diff --git a/spec/ameba/rule/predicate_name_spec.cr b/spec/ameba/rule/predicate_name_spec.cr
index d7d1a24a..169c30dd 100644
--- a/spec/ameba/rule/predicate_name_spec.cr
+++ b/spec/ameba/rule/predicate_name_spec.cr
@@ -44,5 +44,16 @@ module Ameba::Rule
       error.message.should eq(
         "Favour method name 'picture?' over 'has_picture?'")
     end
+
+    it "ignores if alternative name isn't valid syntax" do
+      s = Source.new %q(
+        class Image
+          def is_404?(x)
+            true
+          end
+        end
+      )
+      subject.catch(s).should be_valid
+    end
   end
 end
diff --git a/src/ameba/rule/predicate_name.cr b/src/ameba/rule/predicate_name.cr
index c0b49d37..29e0b581 100644
--- a/src/ameba/rule/predicate_name.cr
+++ b/src/ameba/rule/predicate_name.cr
@@ -1,6 +1,6 @@
 module Ameba::Rule
   # A rule that disallows tautological predicate names, meaning those that
-  # start with the prefix `has_` or the prefix `is_`.
+  # start with the prefix `has_` or the prefix `is_`. Ignores if the alternative isn't valid Crystal code (e.g. `is_404?`).
   #
   # Favour these:
   #
@@ -40,8 +40,11 @@ module Ameba::Rule
 
     def test(source, node : Crystal::Def)
       if node.name =~ /^(is|has)_(\w+)\?/
+        alternative = $2
+        return unless alternative =~ /^[a-z_][a-zA-Z_0-9]*$/
+
         source.error self, node.location,
-          "Favour method name '#{$2}?' over '#{node.name}'"
+          "Favour method name '#{alternative}?' over '#{node.name}'"
       end
     end
   end