diff --git a/prism/util/pm_strncasecmp.c b/prism/util/pm_strncasecmp.c
index 2240bf8110..3f58421554 100644
--- a/prism/util/pm_strncasecmp.c
+++ b/prism/util/pm_strncasecmp.c
@@ -1,5 +1,17 @@
 #include "prism/util/pm_strncasecmp.h"
 
+/**
+ * A locale-insensitive version of `tolower(3)`
+ */
+static inline int
+pm_tolower(int c)
+{
+    if ('A' <= c && c <= 'Z') {
+        return c | 0x20;
+    }
+    return c;
+}
+
 /**
  * Compare two strings, ignoring case, up to the given length. Returns 0 if the
  * strings are equal, a negative number if string1 is less than string2, or a
@@ -16,7 +28,7 @@ pm_strncasecmp(const uint8_t *string1, const uint8_t *string2, size_t length) {
 
     while (offset < length && string1[offset] != '\0') {
         if (string2[offset] == '\0') return string1[offset];
-        if ((difference = tolower(string1[offset]) - tolower(string2[offset])) != 0) return difference;
+        if ((difference = pm_tolower(string1[offset]) - pm_tolower(string2[offset])) != 0) return difference;
         offset++;
     }