diff --git a/src/include/os/alpha/epicsMath.h b/src/include/os/alpha/epicsMath.h
index 88c95b24f..58c72aa0c 100644
--- a/src/include/os/alpha/epicsMath.h
+++ b/src/include/os/alpha/epicsMath.h
@@ -2,6 +2,6 @@
 #define epicsMathh
 
 #include <math.h>
-#define isinf(D) 0
+#define isinf(D) (!finite((D)) && !(isnan((D))))
 
 #endif /* epicsMathh */
diff --git a/src/include/os/vms/epicsMath.h b/src/include/os/vms/epicsMath.h
index 88c95b24f..58c72aa0c 100644
--- a/src/include/os/vms/epicsMath.h
+++ b/src/include/os/vms/epicsMath.h
@@ -2,6 +2,6 @@
 #define epicsMathh
 
 #include <math.h>
-#define isinf(D) 0
+#define isinf(D) (!finite((D)) && !(isnan((D))))
 
 #endif /* epicsMathh */