diff --git a/src/functions/assert/Time/Should-BeFasterThan.ps1 b/src/functions/assert/Time/Should-BeFasterThan.ps1
index 826f2b9dd..ffced49b3 100644
--- a/src/functions/assert/Time/Should-BeFasterThan.ps1
+++ b/src/functions/assert/Time/Should-BeFasterThan.ps1
@@ -76,4 +76,11 @@
         }
         return
     }
+
+    # Neither branch matched. Both branches above return, so reaching here means we were handed
+    # something we cannot measure or compare. Without this the assertion would return having done
+    # nothing and the test would pass, which is the worst way to fail. It also hid a CI flake: a
+    # test asserting that a 10ms sleep is slower than 1ms failed while the whole test took 3ms,
+    # because the scriptblock was never run and nothing said so.
+    $assert.Fail("Expected a [scriptblock] to measure or a [timespan] to compare,<because> but got <actualType> <actual>.", @{ Actual = $Actual; Because = $Because })
 }
diff --git a/src/functions/assert/Time/Should-BeSlowerThan.ps1 b/src/functions/assert/Time/Should-BeSlowerThan.ps1
index 2558597f2..dbc0c4d5a 100644
--- a/src/functions/assert/Time/Should-BeSlowerThan.ps1
+++ b/src/functions/assert/Time/Should-BeSlowerThan.ps1
@@ -83,4 +83,11 @@
         }
         return
     }
+
+    # Neither branch matched. Both branches above return, so reaching here means we were handed
+    # something we cannot measure or compare. Without this the assertion would return having done
+    # nothing and the test would pass, which is the worst way to fail. It also hid a CI flake: a
+    # test asserting that a 10ms sleep is slower than 1ms failed while the whole test took 3ms,
+    # because the scriptblock was never run and nothing said so.
+    $assert.Fail("Expected a [scriptblock] to measure or a [timespan] to compare,<because> but got <actualType> <actual>.", @{ Actual = $Actual; Because = $Because })
 }
diff --git a/tst/functions/assert/Time/Should-BeFasterThan.Tests.ps1 b/tst/functions/assert/Time/Should-BeFasterThan.Tests.ps1
index 11f1e09ce..22189b8b5 100644
--- a/tst/functions/assert/Time/Should-BeFasterThan.Tests.ps1
+++ b/tst/functions/assert/Time/Should-BeFasterThan.Tests.ps1
@@ -60,6 +60,18 @@ Describe "Should-BeFasterThan" {
         $err.Exception.Message | Verify-Like '*because I said so*'
     }
 
+    It "Throws when actual is neither a scriptblock nor a timespan" -ForEach @(
+        @{ Actual = 'a string' }
+        @{ Actual = 42 }
+        @{ Actual = $null }
+    ) {
+        # Without this the assertion returned having done nothing and the test passed. That also
+        # made a CI flake unreadable: a scriptblock that was never run looked like a scriptblock
+        # that ran impossibly fast.
+        $err = { $Actual | Should-BeFasterThan -Expected 1ms } | Verify-AssertionFailed
+        $err.Exception.Message | Verify-Like '*Expected a `[scriptblock`] to measure or a `[timespan`] to compare*'
+    }
+
     It "Requires Expected" {
         # Don't invoke with Expected missing to test this: a missing mandatory parameter makes
         # PowerShell prompt for it, which hangs an interactive test.ps1 run and the release build.
diff --git a/tst/functions/assert/Time/Should-BeSlowerThan.Tests.ps1 b/tst/functions/assert/Time/Should-BeSlowerThan.Tests.ps1
index f2350678e..39d458195 100644
--- a/tst/functions/assert/Time/Should-BeSlowerThan.Tests.ps1
+++ b/tst/functions/assert/Time/Should-BeSlowerThan.Tests.ps1
@@ -35,6 +35,18 @@ Describe "Should-BeSlowerThan" {
         $err.Exception.Message | Verify-Like '*because I said so*'
     }
 
+    It "Throws when actual is neither a scriptblock nor a timespan" -ForEach @(
+        @{ Actual = 'a string' }
+        @{ Actual = 42 }
+        @{ Actual = $null }
+    ) {
+        # Without this the assertion returned having done nothing and the test passed. That also
+        # made a CI flake unreadable: a scriptblock that was never run looked like a scriptblock
+        # that ran impossibly fast.
+        $err = { $Actual | Should-BeSlowerThan -Expected 1ms } | Verify-AssertionFailed
+        $err.Exception.Message | Verify-Like '*Expected a `[scriptblock`] to measure or a `[timespan`] to compare*'
+    }
+
     It "Requires Expected" {
         # Don't invoke with Expected missing to test this: a missing mandatory parameter makes
         # PowerShell prompt for it, which hangs an interactive test.ps1 run and the release build.