diff --git a/lib/dentaku/ast/arithmetic.rb b/lib/dentaku/ast/arithmetic.rb
index 62d54835..81eb9f56 100644
--- a/lib/dentaku/ast/arithmetic.rb
+++ b/lib/dentaku/ast/arithmetic.rb
@@ -7,7 +7,9 @@ module AST
     class Arithmetic < Operation
       def initialize(*)
         super
-        fail "#{ self.class } requires numeric operands" unless valid_node?(left) && valid_node?(right)
+        unless valid_node?(left) && valid_node?(right)
+          fail ParseError, "#{ self.class } requires numeric operands"
+        end
       end
 
       def type
@@ -29,7 +31,7 @@ def cast(value, prefer_integer=true)
       end
 
       def valid_node?(node)
-        node.dependencies.any? || node.type == :numeric
+        node && (node.dependencies.any? || node.type == :numeric)
       end
     end
 
diff --git a/lib/dentaku/ast/combinators.rb b/lib/dentaku/ast/combinators.rb
index 07b0ec7b..a02fd581 100644
--- a/lib/dentaku/ast/combinators.rb
+++ b/lib/dentaku/ast/combinators.rb
@@ -5,7 +5,9 @@ module AST
     class Combinator < Operation
       def initialize(*)
         super
-        fail "#{ self.class } requires logical operands" unless valid_node?(left) && valid_node?(right)
+        unless valid_node?(left) && valid_node?(right)
+          fail ParseError, "#{ self.class } requires logical operands"
+        end
       end
 
       def type
diff --git a/lib/dentaku/ast/function.rb b/lib/dentaku/ast/function.rb
index b37c011d..2f1b82a2 100644
--- a/lib/dentaku/ast/function.rb
+++ b/lib/dentaku/ast/function.rb
@@ -12,7 +12,9 @@ def dependencies(context={})
       end
 
       def self.get(name)
-        registry.fetch(function_name(name)) { fail "Undefined function #{ name } "}
+        registry.fetch(function_name(name)) {
+          fail ParseError, "Undefined function #{ name }"
+        }
       end
 
       def self.register(name, type, implementation)
diff --git a/lib/dentaku/ast/negation.rb b/lib/dentaku/ast/negation.rb
index 39d7010c..bb281b20 100644
--- a/lib/dentaku/ast/negation.rb
+++ b/lib/dentaku/ast/negation.rb
@@ -3,7 +3,7 @@ module AST
     class Negation < Operation
       def initialize(node)
         @node = node
-        fail "Negation requires numeric operand" unless valid_node?(node)
+        fail ParseError, "Negation requires numeric operand" unless valid_node?(node)
       end
 
       def value(context={})
@@ -33,7 +33,7 @@ def dependencies(context={})
       private
 
       def valid_node?(node)
-        node.dependencies.any? || node.type == :numeric
+        node && (node.dependencies.any? || node.type == :numeric)
       end
     end
   end
diff --git a/lib/dentaku/exceptions.rb b/lib/dentaku/exceptions.rb
index 5d1d4643..bcb875e0 100644
--- a/lib/dentaku/exceptions.rb
+++ b/lib/dentaku/exceptions.rb
@@ -7,4 +7,7 @@ def initialize(unbound_variables)
       super("no value provided for variables: #{ unbound_variables.join(', ') }")
     end
   end
+
+  class ParseError < StandardError
+  end
 end
diff --git a/lib/dentaku/parser.rb b/lib/dentaku/parser.rb
index e8fef1ba..40ed11af 100644
--- a/lib/dentaku/parser.rb
+++ b/lib/dentaku/parser.rb
@@ -102,7 +102,7 @@ def parse
             end
 
             unless operations.count == 1 && operations.last == AST::Case
-              fail "Unprocessed token #{ token.value }"
+              fail ParseError, "Unprocessed token #{ token.value }"
             end
             consume(arities.pop.succ)
           when :when
@@ -139,7 +139,7 @@ def parse
 
             operations.push(AST::CaseElse)
           else
-            fail "Unknown case token #{ token.value }"
+            fail ParseError, "Unknown case token #{ token.value }"
           end
 
         when :grouping
@@ -158,7 +158,7 @@ def parse
             end
 
             lparen = operations.pop
-            fail "Unbalanced parenthesis" unless lparen == AST::Grouping
+            fail ParseError, "Unbalanced parenthesis" unless lparen == AST::Grouping
 
             if operations.last && operations.last < AST::Function
               consume(arities.pop.succ)
@@ -171,11 +171,11 @@ def parse
             end
 
           else
-            fail "Unknown grouping token #{ token.value }"
+            fail ParseError, "Unknown grouping token #{ token.value }"
           end
 
         else
-          fail "Not implemented for tokens of category #{ token.category }"
+          fail ParseError, "Not implemented for tokens of category #{ token.category }"
         end
       end
 
@@ -184,7 +184,7 @@ def parse
       end
 
       unless output.count == 1
-        fail "Parse error"
+        fail ParseError, "Invalid statement"
       end
 
       output.first
diff --git a/spec/ast/addition_spec.rb b/spec/ast/addition_spec.rb
index b47df497..90e2b217 100644
--- a/spec/ast/addition_spec.rb
+++ b/spec/ast/addition_spec.rb
@@ -17,7 +17,7 @@
   it 'requires numeric operands' do
     expect {
       described_class.new(five, t)
-    }.to raise_error(RuntimeError, /requires numeric operands/)
+    }.to raise_error(Dentaku::ParseError, /requires numeric operands/)
 
     expression = Dentaku::AST::Multiplication.new(five, five)
     group = Dentaku::AST::Grouping.new(expression)
diff --git a/spec/ast/and_spec.rb b/spec/ast/and_spec.rb
index 9aa2e409..75e2da04 100644
--- a/spec/ast/and_spec.rb
+++ b/spec/ast/and_spec.rb
@@ -17,7 +17,7 @@
   it 'requires logical operands' do
     expect {
       described_class.new(t, five)
-    }.to raise_error(RuntimeError, /requires logical operands/)
+    }.to raise_error(Dentaku::ParseError, /requires logical operands/)
 
     expression = Dentaku::AST::LessThanOrEqual.new(five, five)
     expect {
diff --git a/spec/ast/division_spec.rb b/spec/ast/division_spec.rb
index c4118700..2f532b04 100644
--- a/spec/ast/division_spec.rb
+++ b/spec/ast/division_spec.rb
@@ -17,7 +17,7 @@
   it 'requires numeric operands' do
     expect {
       described_class.new(five, t)
-    }.to raise_error(RuntimeError, /requires numeric operands/)
+    }.to raise_error(Dentaku::ParseError, /requires numeric operands/)
 
     expression = Dentaku::AST::Multiplication.new(five, five)
     group = Dentaku::AST::Grouping.new(expression)
diff --git a/spec/ast/function_spec.rb b/spec/ast/function_spec.rb
index 9410a0df..0f106654 100644
--- a/spec/ast/function_spec.rb
+++ b/spec/ast/function_spec.rb
@@ -7,7 +7,9 @@
   end
 
   it 'raises an exception when trying to access an undefined function' do
-    expect { described_class.get("flarble") }.to raise_error(RuntimeError, /undefined function/i)
+    expect {
+      described_class.get("flarble")
+    }.to raise_error(Dentaku::ParseError, /undefined function/i)
   end
 
   it 'registers a custom function' do
diff --git a/spec/parser_spec.rb b/spec/parser_spec.rb
index 8d083ec7..78cc4a6e 100644
--- a/spec/parser_spec.rb
+++ b/spec/parser_spec.rb
@@ -123,4 +123,14 @@
        case_close]).parse
     expect(node.value(x: 3)).to eq(4)
   end
+
+  it 'raises an error on parse failure' do
+    five  = Dentaku::Token.new(:numeric, 5)
+    times = Dentaku::Token.new(:operator, :multiply)
+    minus = Dentaku::Token.new(:operator, :subtract)
+
+    expect {
+      described_class.new([five, times, minus]).parse
+    }.to raise_error(Dentaku::ParseError)
+  end
 end