diff --git a/std/bigint.d b/std/bigint.d
index b2fcc07142b..33d0eae7ced 100644
--- a/std/bigint.d
+++ b/std/bigint.d
@@ -1541,7 +1541,7 @@ Returns:
     number in upper case.
 
 */
-string toHex(const(BigInt) x) @safe
+string toHex(const(BigInt) x) pure @safe
 {
     import std.array : appender;
     auto outbuff = appender!string();