diff --git a/lib-src/portmixer/src/px_unix_oss.c b/lib-src/portmixer/src/px_unix_oss.c
index 1056284be..9adc6e7c9 100644
--- a/lib-src/portmixer/src/px_unix_oss.c
+++ b/lib-src/portmixer/src/px_unix_oss.c
@@ -44,6 +44,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/ioctl.h>
+#include <ctype.h>
 
 #if defined(HAVE_SYS_SOUNDCARD_H)
 # include <sys/soundcard.h>