lang: Fix crash in lookup function

lang/funcs/collection.go

@@ -512,9 +512,14 @@ var LookupFunc = function.New(&function.Spec{
 		}
 
 		ty := args[0].Type()
-		key := args[1].AsString()
+
 		switch {
 		case ty.IsObjectType():
+			if !args[1].IsKnown() {
+				return args[2].Type(), nil
+			}
+
+			key := args[1].AsString()
 			if ty.HasAttribute(key) {
 				return args[0].GetAttr(key).Type(), nil
 			} else if len(args) == 3 {

lang/funcs/collection_test.go

@@ -1255,6 +1255,25 @@ func TestLookup(t *testing.T) {
 			cty.UnknownVal(cty.String),
 			false,
 		},
+		{
+			[]cty.Value{
+				simpleMap,
+				cty.UnknownVal(cty.String),
+			},
+			cty.UnknownVal(cty.String),
+			false,
+		},
+		{
+			[]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"foo": cty.StringVal("a"),
+					"bar": cty.StringVal("b"),
+				}),
+				cty.UnknownVal(cty.String),
+			},
+			cty.UnknownVal(cty.String),
+			false,
+		},
 	}
 
 	for _, test := range tests {