diff --git a/expression.c b/expression.c
index 502e987..851b544 100644
--- a/expression.c
+++ b/expression.c
@@ -769,8 +769,9 @@ void ExpressionPrefixOperator(struct ParseState *Parser,
                     (void*)((char*)TopValue->Val->Pointer-Size);
                 break;
             case TokenUnaryNot:
+                /* conditionally checking a pointer's value */
                 TopValue->Val->Pointer =
-                    (void*)((char*)(!TopValue->Val->Pointer));
+                    (void*)((TopValue->Val->Pointer) ? NULL : (void*)1);
                 break;
             default:
                 ProgramFail(Parser, "invalid operation");
diff --git a/tests/14_if.c b/tests/14_if.c
index bb56ef4..ecb7e5a 100644
--- a/tests/14_if.c
+++ b/tests/14_if.c
@@ -13,5 +13,23 @@ if (b)
 else
     printf("b is false\n");
 
+int *c = 0;
+if (c)
+    printf("c is true\n");
+else
+    printf("c is false\n");
+if (!c)
+    printf("c is true\n");
+else
+    printf("c is false\n");
+c = &b;
+if (c)
+    printf("c is true\n");
+else
+    printf("c is false\n");
+if (!c)
+    printf("c is true\n");
+else
+    printf("c is false\n");
 
 void main() {}
diff --git a/tests/14_if.expect b/tests/14_if.expect
index c32c415..26f6d95 100644
--- a/tests/14_if.expect
+++ b/tests/14_if.expect
@@ -1,2 +1,6 @@
 a is true
 b is false
+c is false
+c is true
+c is true
+c is false