diff --git a/lib/pure/json.nim b/lib/pure/json.nim
index b53bc3f025..3565e2febe 100644
--- a/lib/pure/json.nim
+++ b/lib/pure/json.nim
@@ -761,7 +761,7 @@ proc `==`* (a,b: JsonNode): bool =
     of JObject:
       a.fields == b.fields
 
-proc hash*(n: Table[string, JsonNode]): Hash
+proc hash*(n: Table[string, JsonNode]): Hash {.noSideEffect.}
 
 proc hash*(n: JsonNode): Hash =
   ## Compute the hash for a JSON node