diff --git a/src/port/isinf.c b/src/port/isinf.c
index 7e8aabc..84aeb74 100644
--- a/src/port/isinf.c
+++ b/src/port/isinf.c
@@ -30,6 +30,7 @@ isinf(double d)
 	switch (type)
 	{
 		case FP_NINF:
+			return -1;
 		case FP_PINF:
 			return 1;
 		default: