diff --git a/media/gmp-clearkey/0.1/ClearKeyUtils.cpp b/media/gmp-clearkey/0.1/ClearKeyUtils.cpp
index 123ca2464cb3..b5bea0a73d1d 100644
--- a/media/gmp-clearkey/0.1/ClearKeyUtils.cpp
+++ b/media/gmp-clearkey/0.1/ClearKeyUtils.cpp
@@ -10,6 +10,8 @@
 #include <vector>
 
 #include "ClearKeyUtils.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Assertions.h"
 #include "mozilla/Endian.h"
 #include "mozilla/NullPtr.h"
 #include "openaes/oaes_lib.h"
@@ -106,7 +108,12 @@ EncodeBase64Web(vector<uint8_t> aBinary, string& aEncoded)
     out[i] += (*data >> (shift + 2)) & sMask;
     shift = (shift + 2) % 8;
 
-    out[i] = sAlphabet[out[i]];
+    // Cast idx to size_t before using it as an array-index,
+    // to pacify clang 'Wchar-subscripts' warning:
+    size_t idx = static_cast<size_t>(out[i]);
+    MOZ_ASSERT(idx < MOZ_ARRAY_LENGTH(sAlphabet),
+               "out of bounds index for 'sAlphabet'");
+    out[i] = sAlphabet[idx];
   }
 
   return true;