diff --git a/source/dyaml/parser.d b/source/dyaml/parser.d
index d5714f4..b570291 100644
--- a/source/dyaml/parser.d
+++ b/source/dyaml/parser.d
@@ -596,6 +596,7 @@ final class Parser
                     // Any hex digits are 1-byte so this works.
                     char[] hex = oldValue[0 .. hexLength];
                     oldValue = oldValue[hexLength .. $];
+                    import std.ascii : isHexDigit;
                     assert(!hex.canFind!(d => !d.isHexDigit),
                             "Scanner must ensure the hex string is valid");