diff --git a/src/sample-acc/test/internal-elem-dedicated.xspec b/src/sample-acc/test/internal-elem-dedicated.xspec
index 756be3e..ed32260 100644
--- a/src/sample-acc/test/internal-elem-dedicated.xspec
+++ b/src/sample-acc/test/internal-elem-dedicated.xspec
@@ -18,15 +18,20 @@
             label="At end of subsection remark, 1 element name in stack"
             test="$myv:tree/section/section/remark/accumulator-after('internal-elem')"
             select="('section')"/>
+        <!--
+            Use of exactly-one() ensures that if $myv:tree yields an empty sequence
+            by mistake, you'll get an error instead of having the accumulator function
+            return empty for the wrong reason.
+        -->
         <x:expect
             label="At end of document, stack is empty"
-            test="$myv:tree/accumulator-after('internal-elem')"
+            test="exactly-one($myv:tree)/accumulator-after('internal-elem')"
             select="()"/>
 
         <!-- Variation: Boolean @test and no @select -->
         <x:expect
             label="At end of document, stack is empty"
-            test="empty($myv:tree/accumulator-after('internal-elem'))"/>
+            test="empty(exactly-one($myv:tree)/accumulator-after('internal-elem'))"/>
     </x:scenario>
 
     <!-- 
diff --git a/src/test/acc-report-inclusion.xspec b/src/test/acc-report-inclusion.xspec
index ad1be02..0f28465 100644
--- a/src/test/acc-report-inclusion.xspec
+++ b/src/test/acc-report-inclusion.xspec
@@ -61,8 +61,13 @@
             </x:variable>
             <x:call function="true"/>
             <x:like label="at:prior-value accumulator values for nodes other than document node"/>
+            <!--
+                Use of exactly-one() ensures that if $av:tree/self::document-node() yields an empty sequence
+                by mistake, you'll get an error instead of having the accumulator function
+                return empty for the wrong reason.
+            -->
             <x:expect label="At start of document node, result is () because there is no prior value"
-                test="$av:tree/self::document-node()/accumulator-before('at:prior-value')"
+                test="exactly-one($av:tree/self::document-node())/accumulator-before('at:prior-value')"
                 select="()"/>
             <x:expect label="At end of document node, prior indent level is 0"
                 test="$av:tree/self::document-node()/accumulator-after('at:prior-value')"