diff --git a/src/common/FFstrbuf.h b/src/common/FFstrbuf.h
index 214f4cdb63..7681299d4a 100644
--- a/src/common/FFstrbuf.h
+++ b/src/common/FFstrbuf.h
@@ -567,7 +567,7 @@ static inline bool ffStrbufInitJsonVal(FFstrbuf* strbuf, yyjson_val* jsonVal) {
 
 // Not `pure`: the caller-supplied `fn` may have side effects
 [[gnu::nonnull(1, 2), nodiscard]] static inline bool ffStrbufEndsWithFn(const FFstrbuf* strbuf, int (*const fn)(int)) {
-    return strbuf->length == 0 ? false : fn(strbuf->chars[strbuf->length - 1]);
+    return strbuf->length == 0 ? false : fn((unsigned char)strbuf->chars[strbuf->length - 1]);
 }
 
 [[gnu::nonnull(1, 2), gnu::pure, nodiscard]] static inline bool ffStrbufEndsWith(const FFstrbuf* strbuf, const FFstrbuf* end) {
diff --git a/src/common/impl/FFstrbuf.c b/src/common/impl/FFstrbuf.c
index 494e487c88..cd0304360d 100644
--- a/src/common/impl/FFstrbuf.c
+++ b/src/common/impl/FFstrbuf.c
@@ -323,7 +323,7 @@ void ffStrbufTrimLeftSpace(FFstrbuf* strbuf) {
     }
 
     uint32_t index = 0;
-    while (index < strbuf->length && isspace(strbuf->chars[index])) {
+    while (index < strbuf->length && isspace((unsigned char)strbuf->chars[index])) {
         ++index;
     }
 
diff --git a/src/common/strutil.h b/src/common/strutil.h
index ee30dbc41c..a4fe126aa5 100644
--- a/src/common/strutil.h
+++ b/src/common/strutil.h
@@ -28,7 +28,7 @@ static inline bool ffStrSet(const char* str) {
         return false;
     }
 
-    while (isspace(*str)) {
+    while (isspace((unsigned char)*str)) {
         str++;
     }
 
diff --git a/tests/strutil.c b/tests/strutil.c
index 1b06ff5b74..4998889542 100644
--- a/tests/strutil.c
+++ b/tests/strutil.c
@@ -26,6 +26,8 @@ int main(void) {
         VERIFY(ffStrSet("a"));
         VERIFY(ffStrSet(" a"));
         VERIFY(ffStrSet("a "));
+        VERIFY(ffStrSet("\x80"));
+        VERIFY(ffStrSet("\xff"));
     }
 
     {