diff --git a/src/context.c b/src/context.c
index a0cf839..0163a3c 100644
--- a/src/context.c
+++ b/src/context.c
@@ -50,7 +50,7 @@ l_vsnprintf (char *str, size_t size, const char *format, va_list ap)
 	int n;
 
 	if (size == 0)
-		return 0;
+		return -1;
 
 #ifdef _MSC_VER
 	/*