diff --git a/extensions/math_ext.cc b/extensions/math_ext.cc
index 4d133d90c..a7773da19 100644
--- a/extensions/math_ext.cc
+++ b/extensions/math_ext.cc
@@ -266,7 +266,11 @@ Value BitShiftLeftInt(int64_t lhs, int64_t rhs) {
   if (rhs > 63) {
     return IntValue(0);
   }
-  return IntValue(lhs << static_cast<int>(rhs));
+  // Shift in the unsigned domain to avoid undefined behaviour when lhs is
+  // negative or the shift moves bits into the sign bit, matching the bit
+  // pattern semantics already used by bitShiftRight.
+  return IntValue(absl::bit_cast<int64_t>(absl::bit_cast<uint64_t>(lhs)
+                                          << static_cast<int>(rhs)));
 }
 
 Value BitShiftLeftUint(uint64_t lhs, int64_t rhs) {
diff --git a/extensions/math_ext_test.cc b/extensions/math_ext_test.cc
index 72605648f..ea9331970 100644
--- a/extensions/math_ext_test.cc
+++ b/extensions/math_ext_test.cc
@@ -563,6 +563,8 @@ INSTANTIATE_TEST_SUITE_P(
          {"math.bitNot(2) == -3"},
          {"math.bitAnd(math.bitNot(0x3u), 0xFFu) == 0xFCu"},
          {"math.bitShiftLeft(1, 1) == 2"},
+         {"math.bitShiftLeft(-1, 1) == -2"},
+         {"math.bitShiftLeft(-4, 2) == -16"},
          {"math.bitShiftLeft(1u, 1) == 2u"},
          {"math.bitShiftRight(4, 1) == 2"},
          {"math.bitShiftRight(4u, 1) == 2u"}}));