diff --git a/auto/cc/test b/auto/cc/test
index 34e4379ea..ac4d5f337 100644
--- a/auto/cc/test
+++ b/auto/cc/test
@@ -67,6 +67,8 @@ case $NXT_CC_NAME in
 
         NXT_CFLAGS="$NXT_CFLAGS -fno-strict-overflow"
 
+        NXT_CFLAGS="$NXT_CFLAGS -funsigned-char"
+
         NXT_CFLAGS="$NXT_CFLAGS -std=gnu11"
 
         NXT_CFLAGS="$NXT_CFLAGS -O"
@@ -109,6 +111,8 @@ case $NXT_CC_NAME in
 
         NXT_CFLAGS="$NXT_CFLAGS -fno-strict-overflow"
 
+        NXT_CFLAGS="$NXT_CFLAGS -funsigned-char"
+
         NXT_CFLAGS="$NXT_CFLAGS -std=gnu11"
 
         NXT_CFLAGS="$NXT_CFLAGS -O"