diff --git a/src/common/impl/FFstrbuf.c b/src/common/impl/FFstrbuf.c
index cd0304360d..23f10ff85d 100644
--- a/src/common/impl/FFstrbuf.c
+++ b/src/common/impl/FFstrbuf.c
@@ -609,13 +609,13 @@ void ffStrbufAppendDouble(FFstrbuf* strbuf, double value, int8_t precision, bool
 
 void ffStrbufUpperCase(FFstrbuf* strbuf) {
     for (uint32_t i = 0; i < strbuf->length; ++i) {
-        strbuf->chars[i] = (char) toupper(strbuf->chars[i]);
+        strbuf->chars[i] = (char) toupper((unsigned char) strbuf->chars[i]);
     }
 }
 
 void ffStrbufLowerCase(FFstrbuf* strbuf) {
     for (uint32_t i = 0; i < strbuf->length; ++i) {
-        strbuf->chars[i] = (char) tolower(strbuf->chars[i]);
+        strbuf->chars[i] = (char) tolower((unsigned char) strbuf->chars[i]);
     }
 }
 
diff --git a/src/common/impl/font.c b/src/common/impl/font.c
index 681c249069..42ec359ed9 100644
--- a/src/common/impl/font.c
+++ b/src/common/impl/font.c
@@ -91,7 +91,7 @@ void ffFontInitQt(FFfont* font, const char* data) {
         goto exit;
     }
     data++;
-    if (isalpha(*data)) {
+    if (isalpha((unsigned char) *data)) {
         do {
             FFstrbuf* style = FF_LIST_ADD(FFstrbuf, font->styles);
             ffStrbufInit(style);
diff --git a/src/common/impl/properties.c b/src/common/impl/properties.c
index 9a4fe290ec..54c1f67c85 100644
--- a/src/common/impl/properties.c
+++ b/src/common/impl/properties.c
@@ -34,7 +34,7 @@ bool ffParsePropLinePointer(const char** line, const char* start, FFstrbuf* buff
         }
 
         // Line doesn't match start, skip it
-        if (tolower(**line) != tolower(*start) || **line == '\0') {
+        if (tolower((unsigned char) **line) != tolower((unsigned char) *start) || **line == '\0') {
             return false;
         }
 
diff --git a/src/detection/chassis/chassis_linux.c b/src/detection/chassis/chassis_linux.c
index e23f73463e..55ad895c9b 100644
--- a/src/detection/chassis/chassis_linux.c
+++ b/src/detection/chassis/chassis_linux.c
@@ -33,7 +33,7 @@ const char* ffDetectChassis(FFChassisResult* result) {
             }
         } else if (ffReadFileBuffer("/sys/firmware/devicetree/base/chassis-type", &result->type) && result->type.length > 0) {
             ffStrbufTrimRight(&result->type, '\0');
-            result->type.chars[0] = (char) toupper(result->type.chars[0]);
+            result->type.chars[0] = (char) toupper((unsigned char) result->type.chars[0]);
         }
     }
     return nullptr;
diff --git a/src/detection/cpu/cpu_linux.c b/src/detection/cpu/cpu_linux.c
index 31c2331037..51887a0438 100644
--- a/src/detection/cpu/cpu_linux.c
+++ b/src/detection/cpu/cpu_linux.c
@@ -465,7 +465,7 @@ static void detectAndroid(FFCPUResult* cpu) {
     } else if (ffStrbufIgnCaseEqualS(&cpu->vendor, "MediaTek")) { // sometimes "Mediatek"
         ffStrbufSetStatic(&cpu->vendor, "MediaTek");
     } else if (cpu->vendor.length > 0) {
-        cpu->vendor.chars[0] = (char) toupper(cpu->vendor.chars[0]);
+        cpu->vendor.chars[0] = (char) toupper((unsigned char) cpu->vendor.chars[0]);
     }
 
     if (ffStrbufEqualS(&cpu->vendor, "Qualcomm") && ffStrbufStartsWithS(&cpu->name, "SM")) {
@@ -1032,18 +1032,18 @@ static const char* detectPhysicalCores(FFCPUResult* cpu) {
         // Raspberry Pi
         ffStrbufSetStatic(&cpu->vendor, "Broadcom");
         for (const char* p = model; *p; ++p) {
-            ffStrbufAppendC(&cpu->name, (char) toupper(*p));
+            ffStrbufAppendC(&cpu->name, (char) toupper((unsigned char) *p));
         }
     } else if (ffStrEquals(vendor, "thead")) {
         // Lichee Pi?
         ffStrbufSetStatic(&cpu->vendor, "T-Head");
         for (const char* p = model; *p; ++p) {
-            ffStrbufAppendC(&cpu->name, (char) toupper(*p));
+            ffStrbufAppendC(&cpu->name, (char) toupper((unsigned char) *p));
         }
     } else {
         ffStrbufSetS(&cpu->name, model);
         ffStrbufSetS(&cpu->vendor, vendor);
-        cpu->vendor.chars[0] = (char) toupper(vendor[0]);
+        cpu->vendor.chars[0] = (char) toupper((unsigned char) vendor[0]);
     }
 }
 
diff --git a/src/detection/wm/wm_apple.m b/src/detection/wm/wm_apple.m
index e02c089bb7..ccee2524f8 100644
--- a/src/detection/wm/wm_apple.m
+++ b/src/detection/wm/wm_apple.m
@@ -71,7 +71,7 @@
         }
 
         ffStrbufAppendS(pluginName, comm);
-        pluginName->chars[0] = (char) toupper(pluginName->chars[0]);
+        pluginName->chars[0] = (char) toupper((unsigned char) pluginName->chars[0]);
         break;
     }
 
diff --git a/src/detection/wmtheme/wmtheme_windows.c b/src/detection/wmtheme/wmtheme_windows.c
index adaece574b..a622117ccd 100644
--- a/src/detection/wmtheme/wmtheme_windows.c
+++ b/src/detection/wmtheme/wmtheme_windows.c
@@ -117,8 +117,8 @@ bool ffDetectWmTheme(FFstrbuf* themeOrError) {
             if (ffRegReadStrbuf(hKey, L"CurrentTheme", &theme, nullptr)) {
                 ffStrbufSubstrBeforeLastC(&theme, '.');
                 ffStrbufSubstrAfterLastC(&theme, '\\');
-                if (isalpha(theme.chars[0])) {
-                    theme.chars[0] = (char) toupper(theme.chars[0]);
+                if (isalpha((unsigned char) theme.chars[0])) {
+                    theme.chars[0] = (char) toupper((unsigned char) theme.chars[0]);
                 }
 
                 ffStrbufAppend(themeOrError, &theme);
diff --git a/src/logo/logo.c b/src/logo/logo.c
index b95ac5fdc1..44d76ce0cf 100644
--- a/src/logo/logo.c
+++ b/src/logo/logo.c
@@ -366,11 +366,11 @@ static bool logoHasName(const FFlogo* logo, const FFstrbuf* name, bool small) {
 }
 
 static const FFlogo* logoGetBuiltin(const FFstrbuf* name, FFLogoSize size) {
-    if (name->length == 0 || !isalpha(name->chars[0])) {
+    if (name->length == 0 || !isalpha((unsigned char) name->chars[0])) {
         return nullptr;
     }
 
-    for (const FFlogo* logo = ffLogoBuiltins[toupper(name->chars[0]) - 'A']; *logo->names; ++logo) {
+    for (const FFlogo* logo = ffLogoBuiltins[toupper((unsigned char) name->chars[0]) - 'A']; *logo->names; ++logo) {
         switch (size) {
             // Never use alternate logos
             case FF_LOGO_SIZE_NORMAL:
diff --git a/src/modules/media/media.c b/src/modules/media/media.c
index 14b7faa9ac..57baf266b5 100644
--- a/src/modules/media/media.c
+++ b/src/modules/media/media.c
@@ -7,7 +7,7 @@
 #include <ctype.h>
 
 static inline bool shouldIgnoreChar(char c) {
-    return isblank(c) || c == '-' || c == '.';
+    return isblank((unsigned char) c) || c == '-' || c == '.';
 }
 
 static bool artistInSongTitle(const FFstrbuf* song, const FFstrbuf* artist) {
@@ -31,7 +31,7 @@ static bool artistInSongTitle(const FFstrbuf* song, const FFstrbuf* artist) {
             return false;
         }
 
-        if (tolower(song->chars[songIndex]) != tolower(artist->chars[artistIndex])) {
+        if (tolower((unsigned char) song->chars[songIndex]) != tolower((unsigned char) artist->chars[artistIndex])) {
             return false;
         }
 
diff --git a/src/modules/packages/packages.c b/src/modules/packages/packages.c
index b8b7f39b2c..7ba7ce1aaa 100644
--- a/src/modules/packages/packages.c
+++ b/src/modules/packages/packages.c
@@ -243,7 +243,7 @@ void ffParsePackagesJsonObject(FFPackagesOptions* options, yyjson_val* module) {
         else if (ffStrEqualsIgnCase(flag, #name)) {             \
             options->disabled |= FF_PACKAGES_FLAG_##name##_BIT; \
         }
-                    switch (toupper(flag[0])) {
+                    switch (toupper((unsigned char) flag[0])) {
                         case 'A':
                             if (false)
                                 ;
diff --git a/src/modules/player/player.c b/src/modules/player/player.c
index f5f4018839..76ea719614 100644
--- a/src/modules/player/player.c
+++ b/src/modules/player/player.c
@@ -49,7 +49,7 @@ bool ffPrintPlayer(FFPlayerOptions* options) {
 
     // If we don't have subdomains, it is usually more pretty to capitalize the first letter.
     if (playerPrettyIsCustom && ffStrbufFirstIndexC(&playerPretty, '.') == playerPretty.length) {
-        playerPretty.chars[0] = (char) toupper(playerPretty.chars[0]);
+        playerPretty.chars[0] = (char) toupper((unsigned char) playerPretty.chars[0]);
     }
 
     if (playerPrettyIsCustom) {
diff --git a/tests/strbuf.c b/tests/strbuf.c
index 06cfe4754c..25a0907587 100644
--- a/tests/strbuf.c
+++ b/tests/strbuf.c
@@ -397,6 +397,14 @@ int main(void) {
     VERIFY(ffStrbufEqualS(&strbuf, "TEST"));
     ffStrbufDestroy(&strbuf);
 
+    // ffStrbufUpperCase & ffStrbufLowerCase
+    ffStrbufInitS(&strbuf, "Hello, World! \x80\xff");
+    ffStrbufUpperCase(&strbuf);
+    VERIFY(ffStrbufEqualS(&strbuf, "HELLO, WORLD! \x80\xff"));
+    ffStrbufLowerCase(&strbuf);
+    VERIFY(ffStrbufEqualS(&strbuf, "hello, world! \x80\xff"));
+    ffStrbufDestroy(&strbuf);
+
     // ffStrbufEnsureFixedLengthFree / empty buffer
     ffStrbufInit(&strbuf);
     ffStrbufEnsureFixedLengthFree(&strbuf, 10);