diff --git a/crypto/riscvcap.c b/crypto/riscvcap.c
index 03e827fa37..b87fe4c94e 100644
--- a/crypto/riscvcap.c
+++ b/crypto/riscvcap.c
@@ -48,7 +48,7 @@ size_t OPENSSL_instrument_bus2(unsigned int *out, size_t cnt, size_t max)
 static void strtoupper(char *str)
 {
     for (char *x = str; *x; ++x)
-        *x = toupper(*x);
+        *x = toupper((unsigned char)*x);
 }
 
 /* parse_env() parses a RISC-V architecture string. An example of such a string
diff --git a/engines/e_loader_attic.c b/engines/e_loader_attic.c
index 84dff6e2c3..154f36cbdd 100644
--- a/engines/e_loader_attic.c
+++ b/engines/e_loader_attic.c
@@ -983,7 +983,7 @@ static OSSL_STORE_LOADER_CTX *file_open_ex
 #ifdef _WIN32
         /* Windows "file:" URIs with a drive letter start with a '/' */
         if (p[0] == '/' && p[2] == ':' && p[3] == '/') {
-            char c = tolower(p[1]);
+            char c = tolower((unsigned char)p[1]);
 
             if (c >= 'a' && c <= 'z') {
                 p++;
diff --git a/providers/implementations/encode_decode/encode_key2text.c b/providers/implementations/encode_decode/encode_key2text.c
index c0c2923285..db0c8abe82 100644
--- a/providers/implementations/encode_decode/encode_key2text.c
+++ b/providers/implementations/encode_decode/encode_key2text.c
@@ -112,7 +112,8 @@ static int print_labeled_bignum(BIO *out, const char *label, const BIGNUM *bn)
             use_sep = 0; /* The first byte on the next line doesn't have a : */
         }
         if (BIO_printf(out, "%s%c%c", use_sep ? ":" : "",
-                       tolower(p[0]), tolower(p[1])) <= 0)
+                       tolower((unsigned char)p[0]),
+                       tolower((unsigned char)p[1])) <= 0)
             goto err;
         ++bytes;
         p += 2;
diff --git a/providers/implementations/storemgmt/file_store.c b/providers/implementations/storemgmt/file_store.c
index 32d03e611b..eea8f2a0f8 100644
--- a/providers/implementations/storemgmt/file_store.c
+++ b/providers/implementations/storemgmt/file_store.c
@@ -235,7 +235,7 @@ static void *file_open(void *provctx, const char *uri)
 #ifdef _WIN32
         /* Windows "file:" URIs with a drive letter start with a '/' */
         if (p[0] == '/' && p[2] == ':' && p[3] == '/') {
-            char c = tolower(p[1]);
+            char c = tolower((unsigned char)p[1]);
 
             if (c >= 'a' && c <= 'z') {
                 p++;