diff --git a/src/decimal.c b/src/decimal.c
index a5bab69..a4ba9d0 100644
--- a/src/decimal.c
+++ b/src/decimal.c
@@ -397,6 +397,7 @@ static php_decimal_success_t php_decimal_do_operation_handler(zend_uchar opcode,
     /* Unsupported op type - return success to avoid casting. */
     if (UNEXPECTED(op == NULL)) {
         php_decimal_operator_not_supported();
+        ZVAL_UNDEF(result);
         return SUCCESS;
     }
 
diff --git a/src/number.c b/src/number.c
index 2970b93..1f9a72c 100644
--- a/src/number.c
+++ b/src/number.c
@@ -105,6 +105,7 @@ static php_decimal_success_t php_decimal_number_do_operation(zend_uchar opcode,
     /* Unsupported operator - return success to avoid casting. */
     if (UNEXPECTED(func == NULL)) {
         php_decimal_operator_not_supported();
+        ZVAL_UNDEF(result);
         return SUCCESS;
     }
 
diff --git a/src/rational.c b/src/rational.c
index 1038135..e4d6686 100644
--- a/src/rational.c
+++ b/src/rational.c
@@ -316,6 +316,7 @@ static php_decimal_success_t php_decimal_rational_do_operation(zend_uchar opcode
     /* Unsupported operator - return success to avoid casting. */
     if (UNEXPECTED(op == NULL)) {
         php_decimal_operator_not_supported();
+        ZVAL_UNDEF(result);
         return SUCCESS;
     }