diff --git a/src/include/missing_math.h b/src/include/missing_math.h
index daff6d1d4..43d47585f 100644
--- a/src/include/missing_math.h
+++ b/src/include/missing_math.h
@@ -32,9 +32,9 @@ extern int isnan(double);
 #endif
 #endif
 
-#ifndef HAVE_DECL_ISINF
+#if (HAVE_DECL_ISINF < 1)
 #ifndef HAVE_ISINF
-#if defined(HAVE_FINITE) && (defined (HAVE_DECL_ISNAN) || defined (HAVE_ISNAN))
+#if defined(HAVE_FINITE) && ((HAVE_DECL_ISNAN < 1) || defined (HAVE_ISNAN))
 #define isinf(x) (!finite(x) && !isnan(x))
 #else
 #ifdef HAVE_IEEEFP_H
diff --git a/src/maths/misc/isinf.c b/src/maths/misc/isinf.c
index a8b1b561e..bf73b4001 100644
--- a/src/maths/misc/isinf.c
+++ b/src/maths/misc/isinf.c
@@ -1,6 +1,6 @@
 #include "ngspice.h"
 
-#ifndef HAVE_DECL_ISINF
+#if (HAVE_DECL_ISINF < 1)
 
 #ifndef HAVE_ISINF
 
diff --git a/src/maths/misc/isnan.c b/src/maths/misc/isnan.c
index 8e36e7b2c..ea6d349b4 100644
--- a/src/maths/misc/isnan.c
+++ b/src/maths/misc/isnan.c
@@ -1,6 +1,6 @@
 #include "ngspice.h"
 
-#ifndef HAVE_DECL_ISNAN
+#if (HAVE_DECL_ISNAN < 1)
 #ifndef HAVE_ISNAN
 
 /* isnan (originally) for SOI devices in MINGW32  hvogt (dev.c) */