diff --git a/spec/oga/xpath/compiler/calls/substring_before_spec.rb b/spec/oga/xpath/compiler/calls/substring_before_spec.rb
index 58a43b3..14013a3 100644
--- a/spec/oga/xpath/compiler/calls/substring_before_spec.rb
+++ b/spec/oga/xpath/compiler/calls/substring_before_spec.rb
@@ -4,27 +4,38 @@ describe Oga::XPath::Compiler do
   describe 'substring-before() function' do
     before do
       @document = parse('<root><a>-</a><b>a-b-c</b></root>')
+
+      @a1 = @document.children[0].children[0]
     end
 
-    it 'returns the substring of the 1st string before the 2nd string' do
-      evaluate_xpath(@document, 'substring-before("a-b-c", "-")').should == 'a'
+    describe 'at the top-level' do
+      it 'returns the substring of the 1st string before the 2nd string' do
+        evaluate_xpath(@document, 'substring-before("a-b-c", "-")').should == 'a'
+      end
+
+      it 'returns an empty string if the 2nd string is not present' do
+        evaluate_xpath(@document, 'substring-before("a-b-c", "x")').should == ''
+      end
+
+      it 'returns the substring of the 1st node set before the 2nd string' do
+        evaluate_xpath(@document, 'substring-before(root/b, "-")').should == 'a'
+      end
+
+      it 'returns the substring of the 1st node set before the 2nd node set' do
+        evaluate_xpath(@document, 'substring-before(root/b, root/a)')
+          .should == 'a'
+      end
+
+      it 'returns an empty string when using two empty strings' do
+        evaluate_xpath(@document, 'substring-before("", "")').should == ''
+      end
     end
 
-    it 'returns an empty string if the 2nd string is not present' do
-      evaluate_xpath(@document, 'substring-before("a-b-c", "x")').should == ''
-    end
-
-    it 'returns the substring of the 1st node set before the 2nd string' do
-      evaluate_xpath(@document, 'substring-before(root/b, "-")').should == 'a'
-    end
-
-    it 'returns the substring of the 1st node set before the 2nd node set' do
-      evaluate_xpath(@document, 'substring-before(root/b, root/a)')
-        .should == 'a'
-    end
-
-    it 'returns an empty string when using two empty strings' do
-      evaluate_xpath(@document, 'substring-before("", "")').should == ''
+    describe 'in a predicate' do
+      it 'returns a NodeSet containing all matching nodes' do
+        evaluate_xpath(@document, 'root/a[substring-before("foo-bar", "-")]')
+          .should == node_set(@a1)
+      end
     end
   end
 end