diff --git a/lib/if2ip.c b/lib/if2ip.c
index 249be7ce2988fe1bc8441b388c2af75f879c02a9..627422b944399ba914c286f69f718496b4ad429b 100644
--- a/lib/if2ip.c
+++ b/lib/if2ip.c
@@ -32,8 +32,8 @@
 #include <unistd.h>
 #endif
 
-#if ! defined(WIN32) && ! defined(__BEOS__) && !defined(__CYGWIN32__) && \
-    ! defined(__riscos__)
+#if !defined(WIN32) && !defined(__BEOS__) && !defined(__CYGWIN32__) && \
+    !defined(__riscos__) && !defined(__INTERIX)
 
 #ifdef HAVE_SYS_SOCKET_H
 #include <sys/socket.h>
diff --git a/lib/if2ip.h b/lib/if2ip.h
index 566d68ba824dfe6cb8b3a2eb2469c6dcd7fce0c1..239087d74d4fcf463d0b19482cbac7c9bfdd0e01 100644
--- a/lib/if2ip.h
+++ b/lib/if2ip.h
@@ -24,8 +24,8 @@
  ***************************************************************************/
 #include "setup.h"
 
-#if ! defined(WIN32) && ! defined(__BEOS__) && !defined(__CYGWIN32__) && \
-    ! defined(__riscos__)
+#if !defined(WIN32) && !defined(__BEOS__) && !defined(__CYGWIN32__) && \
+    !defined(__riscos__) !defined(__INTERIX)
 extern char *Curl_if2ip(char *interface, char *buf, int buf_size);
 #else
 #define Curl_if2ip(a,b,c) NULL