diff --git a/src/options.c b/src/options.c
index 84b9d2bf..d75764ab 100644
--- a/src/options.c
+++ b/src/options.c
@@ -176,7 +176,8 @@ static void mi_option_init(mi_option_desc_t* desc) {
   #pragma warning(suppress:4996)
   char* s = getenv(buf);
   if (s == NULL) {
-    for (size_t i = 0; i < strlen(buf); i++) {
+    size_t buf_size = strlen(buf);
+    for (size_t i = 0; i < buf_size; i++) {
       buf[i] = toupper(buf[i]);
     }
     #pragma warning(suppress:4996)
@@ -184,7 +185,8 @@ static void mi_option_init(mi_option_desc_t* desc) {
   }
   if (s != NULL) {
     mi_strlcpy(buf, s, sizeof(buf));
-    for (size_t i = 0; i < strlen(buf); i++) {
+    size_t buf_size = strlen(buf); // TODO: use strnlen?
+    for (size_t i = 0; i < buf_size; i++) {
       buf[i] = toupper(buf[i]);
     }
     if (buf[0]==0 || strstr("1;TRUE;YES;ON", buf) != NULL) {