diff --git a/src/Analyser/ExprHandler/AssignHandler.php b/src/Analyser/ExprHandler/AssignHandler.php
index 51995ac6657..fcc12f5b12b 100644
--- a/src/Analyser/ExprHandler/AssignHandler.php
+++ b/src/Analyser/ExprHandler/AssignHandler.php
@@ -241,6 +241,12 @@ public function processAssignVar(
 				$assignedExpr = $this->unwrapAssign($assignedExpr);
 				$type = $scopeBeforeAssignEval->getType($assignedExpr);
 
+				$nativeType = $scope->getNativeType($assignedExpr);
+				if ($isAssignOp && $type instanceof ErrorType) {
+					$type = $scopeBeforeAssignEval->getType($var);
+					$nativeType = $scopeBeforeAssignEval->getNativeType($var);
+				}
+
 				$conditionalExpressions = [];
 				if ($assignedExpr instanceof Ternary) {
 					$if = $assignedExpr->if;
@@ -314,7 +320,7 @@ public function processAssignVar(
 				}
 
 				$nodeScopeResolver->callNodeCallback($nodeCallback, new VariableAssignNode($var, $assignedExpr), $scopeBeforeAssignEval, $storage);
-				$scope = $scope->assignVariable($var->name, $type, $scope->getNativeType($assignedExpr), TrinaryLogic::createYes());
+				$scope = $scope->assignVariable($var->name, $type, $nativeType, TrinaryLogic::createYes());
 				foreach ($conditionalExpressions as $exprString => $holders) {
 					$scope = $scope->addConditionalExpressions($exprString, $holders);
 				}
@@ -414,6 +420,28 @@ public function processAssignVar(
 
 			$valueToWrite = $scope->getType($assignedExpr);
 			$nativeValueToWrite = $scope->getNativeType($assignedExpr);
+
+			if ($isAssignOp && $valueToWrite instanceof ErrorType) {
+				$originalType = $scope->getType($var);
+				foreach ($offsetTypes as [$offsetType]) {
+					if ($offsetType === null) {
+						break;
+					}
+					$originalType = $originalType->getOffsetValueType($offsetType);
+				}
+				$valueToWrite = $originalType;
+			}
+			if ($isAssignOp && $nativeValueToWrite instanceof ErrorType) {
+				$originalNativeType = $scope->getNativeType($var);
+				foreach ($offsetNativeTypes as [$offsetNativeType]) {
+					if ($offsetNativeType === null) {
+						break;
+					}
+					$originalNativeType = $originalNativeType->getOffsetValueType($offsetNativeType);
+				}
+				$nativeValueToWrite = $originalNativeType;
+			}
+
 			$scopeBeforeAssignEval = $scope;
 
 			// 3. eval assigned expr
@@ -781,6 +809,21 @@ public function processAssignVar(
 			$varType = $scope->getType($var);
 			$varNativeType = $scope->getNativeType($var);
 
+			if ($isAssignOp && $valueToWrite instanceof ErrorType) {
+				$originalType = $varType;
+				foreach ($offsetTypes as [$offsetType]) {
+					$originalType = $originalType->getOffsetValueType($offsetType);
+				}
+				$valueToWrite = $originalType;
+			}
+			if ($isAssignOp && $nativeValueToWrite instanceof ErrorType) {
+				$originalNativeType = $varNativeType;
+				foreach ($offsetNativeTypes as [$offsetNativeType]) {
+					$originalNativeType = $originalNativeType->getOffsetValueType($offsetNativeType);
+				}
+				$nativeValueToWrite = $originalNativeType;
+			}
+
 			$offsetValueType = $varType;
 			$offsetNativeValueType = $varNativeType;
 			$offsetValueTypeStack = [$offsetValueType];
diff --git a/tests/PHPStan/Analyser/nsrt/bug-10349.php b/tests/PHPStan/Analyser/nsrt/bug-10349.php
new file mode 100644
index 00000000000..2db8674eb0c
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-10349.php
@@ -0,0 +1,34 @@
+<?php declare(strict_types = 1);
+
+namespace Bug10349Nsrt;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	/**
+	 * @param array<string, array<string, bool|float|int|string>> $expected
+	 */
+	public function testTypePreservationAfterErrorAssignOp(array $expected, int $ptr, string $key): void
+	{
+		assertType('array<string, array<string, bool|float|int|string>>', $expected);
+
+		$expected[$key]['number-1'] += $ptr;
+
+		// After += with ErrorType result, the array type should be preserved
+		assertType('bool|float|int|string', $expected[$key]['number-2']);
+	}
+
+	/**
+	 * @param array<string, bool|float|int|string> $arr
+	 */
+	public function testSimpleArrayTypePreservation(array $arr, int $ptr): void
+	{
+		assertType('bool|float|int|string', $arr['a']);
+
+		$arr['a'] += $ptr;
+
+		// After += with ErrorType result, sibling keys should keep their type
+		assertType('bool|float|int|string', $arr['b']);
+	}
+}
diff --git a/tests/PHPStan/Rules/Operators/InvalidBinaryOperationRuleTest.php b/tests/PHPStan/Rules/Operators/InvalidBinaryOperationRuleTest.php
index 735f7577116..64694f3b2d1 100644
--- a/tests/PHPStan/Rules/Operators/InvalidBinaryOperationRuleTest.php
+++ b/tests/PHPStan/Rules/Operators/InvalidBinaryOperationRuleTest.php
@@ -835,4 +835,86 @@ public function testBug14080(): void
 		$this->analyse([__DIR__ . '/data/bug-14080.php'], []);
 	}
 
+	public function testBug10349(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-10349.php'], [
+			[
+				'Binary operation "+=" between bool|float|int|string and int results in an error.',
+				15,
+			],
+			[
+				'Binary operation "+=" between bool|float|int|string and int results in an error.',
+				20,
+			],
+			[
+				'Binary operation "+=" between bool|float|int|string and int results in an error.',
+				37,
+			],
+			[
+				'Binary operation "+=" between bool|float|int|string and int results in an error.',
+				47,
+			],
+			[
+				'Binary operation "+=" between bool|float|int|string and int results in an error.',
+				49,
+			],
+			[
+				'Binary operation "+=" between bool|float|int|string and int results in an error.',
+				57,
+			],
+			[
+				'Binary operation "+=" between bool|float|int|string and int results in an error.',
+				59,
+			],
+			[
+				'Binary operation "-=" between bool|float|int|string and int results in an error.',
+				67,
+			],
+			[
+				'Binary operation "-=" between bool|float|int|string and int results in an error.',
+				68,
+			],
+			[
+				'Binary operation "*=" between bool|float|int|string and int results in an error.',
+				69,
+			],
+			[
+				'Binary operation "*=" between bool|float|int|string and int results in an error.',
+				70,
+			],
+			[
+				'Binary operation ".=" between array<int>|int and \'foo\' results in an error.',
+				78,
+			],
+			[
+				'Binary operation ".=" between array<int>|int and \'foo\' results in an error.',
+				79,
+			],
+			[
+				'Binary operation "/=" between bool|float|int|string and int results in an error.',
+				87,
+			],
+			[
+				'Binary operation "/=" between bool|float|int|string and int results in an error.',
+				88,
+			],
+			[
+				'Binary operation "%=" between bool|float|int|string and int results in an error.',
+				89,
+			],
+			[
+				'Binary operation "%=" between bool|float|int|string and int results in an error.',
+				90,
+			],
+			[
+				'Binary operation "<<=" between bool|float|int|string and int results in an error.',
+				98,
+			],
+			[
+				'Binary operation "<<=" between bool|float|int|string and int results in an error.',
+				99,
+			],
+		]);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Operators/data/bug-10349.php b/tests/PHPStan/Rules/Operators/data/bug-10349.php
new file mode 100644
index 00000000000..1404d8ed612
--- /dev/null
+++ b/tests/PHPStan/Rules/Operators/data/bug-10349.php
@@ -0,0 +1,101 @@
+<?php declare(strict_types = 1);
+
+namespace Bug10349;
+
+class Foo
+{
+	/**
+	 * @param array<string, array<string, bool|float|int|string>> $expected
+	 */
+	public function issue1A(array $expected, int $ptr): void
+	{
+		foreach ($expected as $key => $param) {
+			if ($param['number-1'] !== false) {
+				// This gets flagged
+				$expected[$key]['number-1'] += $ptr;
+			}
+
+			if ($param['number-2'] !== false) {
+				// This should also get flagged but doesn't
+				$expected[$key]['number-2'] += $ptr;
+			}
+		}
+	}
+
+	/**
+	 * @param array<string, array<string, bool|float|int|string>> $expected
+	 */
+	public function issue1B(array $expected, int $ptr): void
+	{
+		foreach ($expected as $key => $param) {
+			if (is_int($expected[$key]['number-1'])) {
+				$expected[$key]['number-1'] += $ptr;
+			}
+
+			// Even after fixing the first, the second one still doesn't get flagged
+			if ($param['number-2'] !== false) {
+				$expected[$key]['number-2'] += $ptr;
+			}
+		}
+	}
+
+	/**
+	 * @param array<string, array<string, bool|float|int|string>> $expected
+	 */
+	public function multipleOpsNoLoop(array $expected, int $ptr, string $key): void
+	{
+		$expected[$key]['number-1'] += $ptr;
+		// After the first += corrupts the array type, this should still be flagged
+		$expected[$key]['number-2'] += $ptr;
+	}
+
+	/**
+	 * @param array<string, bool|float|int|string> $arr
+	 */
+	public function simpleArray(array $arr, int $ptr): void
+	{
+		$arr['a'] += $ptr;
+		// After the first += corrupts the array type, this should still be flagged
+		$arr['b'] += $ptr;
+	}
+
+	/**
+	 * @param array<string, bool|float|int|string> $arr
+	 */
+	public function otherAssignOps(array $arr, int $ptr): void
+	{
+		$arr['a'] -= $ptr;
+		$arr['b'] -= $ptr;
+		$arr['c'] *= $ptr;
+		$arr['d'] *= $ptr;
+	}
+
+	/**
+	 * @param array<string, array<int>|int> $arr
+	 */
+	public function concatAssignOps(array $arr): void
+	{
+		$arr['a'] .= 'foo';
+		$arr['b'] .= 'foo';
+	}
+
+	/**
+	 * @param array<string, bool|float|int|string> $arr
+	 */
+	public function divAndModAssignOps(array $arr, int $ptr): void
+	{
+		$arr['a'] /= $ptr;
+		$arr['b'] /= $ptr;
+		$arr['c'] %= $ptr;
+		$arr['d'] %= $ptr;
+	}
+
+	/**
+	 * @param array<string, bool|float|int|string> $arr
+	 */
+	public function bitwiseAssignOps(array $arr, int $ptr): void
+	{
+		$arr['a'] <<= $ptr;
+		$arr['b'] <<= $ptr;
+	}
+}