diff --git a/libnixf/src/Basic/diagnostic.py b/libnixf/src/Basic/diagnostic.py
index 8261d8548..58f82e9cc 100644
--- a/libnixf/src/Basic/diagnostic.py
+++ b/libnixf/src/Basic/diagnostic.py
@@ -87,6 +87,12 @@ class Diagnostic(TypedDict):
         "severity": "Error",
         "message": "extra `@` for lambda arg",
     },
+    {
+        "sname": "parse-operator-noassoc",
+        "cname": "OperatorNotAssociative",
+        "severity": "Error",
+        "message": "operator is non-associative",
+    },
     {
         "sname": "let-dynamic",
         "cname": "LetDynamic",
diff --git a/libnixf/src/Parse/ParseOp.cpp b/libnixf/src/Parse/ParseOp.cpp
index 45e7f0b77..0e842bb32 100644
--- a/libnixf/src/Parse/ParseOp.cpp
+++ b/libnixf/src/Parse/ParseOp.cpp
@@ -3,6 +3,7 @@
 
 #include "Parser.h"
 
+#include "nixf/Basic/Diagnostic.h"
 #include "nixf/Basic/Nodes/Op.h"
 
 #include <cassert>
@@ -115,7 +116,9 @@ std::shared_ptr<Expr> Parser::parseExprOpBP(unsigned LeftRBP) {
         if (LeftRBP > LBP)
           return Prefix;
         if (LeftRBP == LBP) {
-          // TODO: noassoc
+          // Report error, operator OP and expr_op is not associative.
+          Diags.emplace_back(Diagnostic::DK_OperatorNotAssociative,
+                             Tok.range());
         }
         consume();
         assert(LastToken && "consume() should have set LastToken");