diff --git a/csim.c b/csim.c
index 4ca764e..12308f3 100644
--- a/csim.c
+++ b/csim.c
@@ -17,11 +17,14 @@ void print_usage()
 
 int parse_int_arg(char* arg)
 {
-	int i = atoi(arg);
-	if (i != 0)
-		return i;
-	else
-		exit(-1);
+	char* end;
+	int i = strtol(arg, &end, 0);
+	if (!i || *end != '\0')
+	{
+		printf("Invalid argument \"%s\"\n", arg);
+		print_usage();
+	}
+	return i;
 }
 
 int main(int argc, char* argv[])