diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index f4c8bdf9431..8bfc71315f9 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -13629,6 +13629,14 @@ omp_instantiate_mapper (gimple_seq *pre_p,
 	  continue;
 	}
 
+      if (OMP_CLAUSE_HAS_ITERATORS (clause))
+	{
+	  sorry_at (OMP_CLAUSE_LOCATION (clause),
+		    "user-defined mapper that uses a %<map%> clause "
+		    "with %<iterator%>");
+	  continue;
+	}
+
       tree decl = OMP_CLAUSE_DECL (clause);
       tree unshared, type;
       bool nonunit_array_with_mapper = false;
diff --git a/gcc/testsuite/c-c++-common/gomp/pr122866.c b/gcc/testsuite/c-c++-common/gomp/pr122866.c
new file mode 100644
index 00000000000..bb42bc634db
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/gomp/pr122866.c
@@ -0,0 +1,21 @@
+/* { dg-do "compile" } */
+
+struct test { int x; };
+
+#pragma omp begin declare target
+void froggify (struct test);
+#pragma omp end declare target
+
+#pragma omp declare mapper(struct test v) map(iterator(i = 0:1), tofrom: v.x)
+/* { dg-message "sorry, unimplemented: user-defined mapper that uses a .map. clause with .iterator." "" { target *-*-* } .-1 } */
+
+int
+main ()
+{
+  struct test s;
+  s.x = -1;
+  #pragma omp target
+  {
+    froggify (s);
+  }
+}