diff --git a/src/openvpn/crypto_backend.h b/src/openvpn/crypto_backend.h
index d119442f..1d206a8c 100644
--- a/src/openvpn/crypto_backend.h
+++ b/src/openvpn/crypto_backend.h
@@ -526,7 +526,7 @@ const char *md_kt_name(const md_kt_t *kt);
  *
  * @return              Message digest size, in bytes, or 0 if ctx was NULL.
  */
-int md_kt_size(const md_kt_t *kt);
+unsigned char md_kt_size(const md_kt_t *kt);
 
 
 /*
diff --git a/src/openvpn/crypto_mbedtls.c b/src/openvpn/crypto_mbedtls.c
index 648a988e..3e77fa9e 100644
--- a/src/openvpn/crypto_mbedtls.c
+++ b/src/openvpn/crypto_mbedtls.c
@@ -823,7 +823,7 @@ md_kt_name(const mbedtls_md_info_t *kt)
     return mbedtls_md_get_name(kt);
 }
 
-int
+unsigned char
 md_kt_size(const mbedtls_md_info_t *kt)
 {
     if (NULL == kt)
diff --git a/src/openvpn/crypto_openssl.c b/src/openvpn/crypto_openssl.c
index 520e40ee..a81dcfd8 100644
--- a/src/openvpn/crypto_openssl.c
+++ b/src/openvpn/crypto_openssl.c
@@ -930,10 +930,10 @@ md_kt_name(const EVP_MD *kt)
     return EVP_MD_name(kt);
 }
 
-int
+unsigned char
 md_kt_size(const EVP_MD *kt)
 {
-    return EVP_MD_size(kt);
+    return (unsigned char)EVP_MD_size(kt);
 }