diff --git a/src/cc65/typeconv.c b/src/cc65/typeconv.c
index 612aa0213..5af7e513c 100644
--- a/src/cc65/typeconv.c
+++ b/src/cc65/typeconv.c
@@ -144,7 +144,16 @@ static int DoConversion (ExprDesc* Expr, int k, type* NewType)
                 /* If the new type is signed, sign extend the value */
                 if (!IsSignUnsigned (NewType)) {
                     if (Expr->ConstVal & (0x01UL << (NewBits-1))) {
-                        Expr->ConstVal |= ((~0L) << NewBits);
+                        /* Beware: NewBits may be 32, in which case a shift
+                         * creates undefined behaviour if a long does also
+                         * have 32 bits. So apply a somewhat complex special
+                         * handling.
+                         */
+                        unsigned long SignBits = ~0UL;
+                        SignBits <<= (NewBits / 2);
+                        NewBits -= (NewBits / 2);
+                        SignBits <<= NewBits;
+                        Expr->ConstVal |= SignBits;
                     }
                 }
             }