diff --git a/source/FluxBase-Core-Tests/FBTest.class.st b/source/FluxBase-Core-Tests/FBTest.class.st
index 75e16fe..e2756d2 100644
--- a/source/FluxBase-Core-Tests/FBTest.class.st
+++ b/source/FluxBase-Core-Tests/FBTest.class.st
@@ -1,6 +1,20 @@
+"
+The base of every FluxBase-Core test. FBFlow class>>transitionClass is a class-side
+variable, and ApptiveBase's tests set it to AGFlowTransition for their runs; their bases
+set it back in #tearDown, but a run that was aborted, or a test base without that
+teardown, leaves it standing - and every FluxBase test that builds a flow afterwards gets
+ApptiveBase transitions and fails far from the cause. So every FluxBase test starts from
+the default, whatever the image was left with.
+"
 Class {
 	#name : 'FBTest',
 	#superclass : 'TestCase',
 	#category : 'FluxBase-Core-Tests',
 	#package : 'FluxBase-Core-Tests'
 }
+
+{ #category : 'running' }
+FBTest >> setUp [
+	super setUp.
+	FBFlow transitionClass: FBFlow defaultTransitionClass
+]