diff --git a/lib/method_source/code_helpers.rb b/lib/method_source/code_helpers.rb
index 4185c18..211e123 100644
--- a/lib/method_source/code_helpers.rb
+++ b/lib/method_source/code_helpers.rb
@@ -127,6 +127,7 @@ module IncompleteExpression
         /embedded document meets end of file/, # =begin
         /unterminated (quoted string|string|regexp|list) meets end of file/, # "quoted string" is ironruby
         /can't find string ".*" anywhere before EOF/, # rbx and jruby
+        /expected a closing delimiter for the `%x` or backtick string/, # prism
         /missing 'end' for/, /expecting kWHEN/ # rbx
       ]
 
diff --git a/spec/method_source/code_helpers_spec.rb b/spec/method_source/code_helpers_spec.rb
index 287ff89..e6115c2 100644
--- a/spec/method_source/code_helpers_spec.rb
+++ b/spec/method_source/code_helpers_spec.rb
@@ -5,6 +5,11 @@
     @tester = Object.new.extend(MethodSource::CodeHelpers)
   end
 
+  it "recognizes incomplete command-string delimiters" do
+    expect(@tester.complete_expression?("%x[word")).to be_falsy
+    expect(@tester.complete_expression?("`word")).to be_falsy
+  end
+
   [
     ["p = '", "'"],
     ["def", "a", "(); end"],