diff --git a/src/units.c b/src/units.c
index 584b048..ed1ea60 100644
--- a/src/units.c
+++ b/src/units.c
@@ -58,6 +58,7 @@
 #include <stdint.h>
 #endif
 #include <sys/socket.h>
+#include <sys/types.h>
 #include <sys/time.h>
 #include <netinet/tcp.h>
 
@@ -261,7 +262,7 @@ extern    "C"
 	{
 	    inNum *= 8;
 	}
-	switch    (toupper(inFormat))
+	switch    (toupper((u_char)inFormat))
 	{
 	case 'B':
 	    conv = UNIT_CONV;