diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index fad3207..8f561a7 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -780,7 +780,7 @@ module Oga
           convert = process(expression, context)
 
           if convert.is_a?(XML::NodeSet)
-            convert = convert.first
+            convert = convert[0]
           end
         else
           convert = current_node
@@ -815,9 +815,28 @@ module Oga
       # @return [Float]
       #
       def on_call_number(context, expression = nil)
-        str_val = on_call_string(context, expression)
+        convert = nil
 
-        return Float(str_val) rescue Float::NAN
+        if expression
+          exp_retval = process(expression, context)
+
+          if exp_retval.is_a?(XML::NodeSet)
+            convert = first_node_text(exp_retval)
+
+          elsif exp_retval == true
+            convert = 1.0
+
+          elsif exp_retval == false
+            convert = 0.0
+
+          elsif exp_retval
+            convert = exp_retval
+          end
+        else
+          convert = current_node.text
+        end
+
+        return Float(convert) rescue Float::NAN
       end
 
       ##
diff --git a/spec/oga/xpath/evaluator/calls/number_spec.rb b/spec/oga/xpath/evaluator/calls/number_spec.rb
index 7f856e1..231ec6d 100644
--- a/spec/oga/xpath/evaluator/calls/number_spec.rb
+++ b/spec/oga/xpath/evaluator/calls/number_spec.rb
@@ -15,6 +15,14 @@ describe Oga::XPath::Evaluator do
       @evaluator.evaluate('number("10.5")').should == 10.5
     end
 
+    example 'convert boolean true to a number' do
+      @evaluator.evaluate('number(true())').should == 1.0
+    end
+
+    example 'convert boolean false to a number' do
+      @evaluator.evaluate('number(false())').should be_zero
+    end
+
     example 'convert a node set to a number' do
       @evaluator.evaluate('number(root/a)').should == 10.0
     end
@@ -30,5 +38,9 @@ describe Oga::XPath::Evaluator do
     example 'return NaN for values that can not be converted to floats' do
       @evaluator.evaluate('number("a")').should be_nan
     end
+
+    example 'return NaN for empty node sets' do
+      @evaluator.evaluate('number(foo)').should be_nan
+    end
   end
 end