diff --git a/config/flay.yml b/config/flay.yml
index 2a5e50be..1f473d54 100644
--- a/config/flay.yml
+++ b/config/flay.yml
@@ -1,3 +1,3 @@
 ---
 threshold: 18
-total_score: 1020
+total_score: 1039
diff --git a/lib/mutant/expression.rb b/lib/mutant/expression.rb
index f6a81d8d..79ccc5b8 100644
--- a/lib/mutant/expression.rb
+++ b/lib/mutant/expression.rb
@@ -17,6 +17,9 @@ module Mutant
 
     REGISTRY = {}
 
+    class InvalidExpressionError < RuntimeError; end
+    class AmbigousExpressionError < RuntimeError; end
+
     # Initialize expression
     #
     # @param [MatchData] match
@@ -88,7 +91,7 @@ module Mutant
     # @api private
     #
     def self.parse(input)
-      try_parse(input) or raise "Expression: #{input.inspect} is not valid"
+      try_parse(input) or fail InvalidExpressionError, "Expression: #{input.inspect} is not valid"
     end
 
     # Parse input into expression
@@ -110,7 +113,7 @@ module Mutant
       when 1
         expressions.first
       else
-        fail "Ambigous expression: #{input.inspect}"
+        fail AmbigousExpressionError, "Ambigous expression: #{input.inspect}"
       end
     end
 
diff --git a/spec/unit/mutant/expression_spec.rb b/spec/unit/mutant/expression_spec.rb
new file mode 100644
index 00000000..5a69be6a
--- /dev/null
+++ b/spec/unit/mutant/expression_spec.rb
@@ -0,0 +1,55 @@
+require 'spec_helper'
+
+describe Mutant::Expression do
+  let(:object) { described_class }
+
+  describe '.try_parse' do
+    subject { object.try_parse(input) }
+
+    context 'on nonsense' do
+      let(:input) { 'foo bar' }
+
+      it { should be(nil) }
+    end
+
+    context 'on a valid expression' do
+      let(:input) { 'Foo' }
+
+      it { should eql(Mutant::Expression::Namespace::Exact.new('Foo')) }
+    end
+
+    context 'on ambigious expression' do
+      class ExpressionA < Mutant::Expression
+        register(/\Atest-syntax\z/)
+      end
+
+      class ExpressionB < Mutant::Expression
+        register(/^test-syntax$/)
+      end
+
+      let(:input) { 'test-syntax' }
+
+      it 'raises an exception' do
+        expect { subject }.to raise_error(Mutant::Expression::AmbigousExpressionError, 'Ambigous expression: "test-syntax"')
+      end
+    end
+  end
+
+  describe '.parse' do
+    subject { object.parse(input) }
+
+    context 'on nonsense' do
+      let(:input) { 'foo bar' }
+
+      it 'raises an exception' do
+        expect { subject }.to raise_error(Mutant::Expression::InvalidExpressionError, 'Expression: "foo bar" is not valid')
+      end
+    end
+
+    context 'on a valid expression' do
+      let(:input) { 'Foo' }
+
+      it { should eql(Mutant::Expression::Namespace::Exact.new('Foo')) }
+    end
+  end
+end