diff --git a/modules/FiniteSetsExt.tla b/modules/FiniteSetsExt.tla
index 7b87b66..1460089 100644
--- a/modules/FiniteSetsExt.tla
+++ b/modules/FiniteSetsExt.tla
@@ -36,7 +36,7 @@ ReduceSet(op(_, _), set, acc) ==
    (* An alias for FoldSet. ReduceSet was used instead of FoldSet in        *)
    (* earlier versions of the community modules.                            *)
    (*************************************************************************)
-   FoldSet(op, set, acc)
+   FoldSet(op, acc, set)
 
 
 FlattenSet(S) ==
diff --git a/tests/FiniteSetsExtTests.tla b/tests/FiniteSetsExtTests.tla
index 5a9481a..4c53a3a 100644
--- a/tests/FiniteSetsExtTests.tla
+++ b/tests/FiniteSetsExtTests.tla
@@ -90,7 +90,6 @@ ASSUME FoldSet(LAMBDA x,y : x + y, 0, 0 .. 10) = 55
 
 \* Without the corresponding Java module override, this overflows TLC's stack.
 ASSUME FoldSet(LAMBDA x,y : x + y, 0, 0 .. 10000) = 50005000
-
 -----------------------------------------------------------------------------
 
 ASSUME ChooseUnique({2, 3, 4, 5}, LAMBDA x : x % 3 = 1) = 4
@@ -122,6 +121,8 @@ ASSUME SymDiff({2,3}, {2,3,4}) = {4}
 ASSUME SumSet(1..3) = 6
 ASSUME ProductSet(1..4) = 24
 
+ASSUME ReduceSet(+, 1..5, 42) = 57
+
 -----------------------------------------------------------------------------
 
 (* TLC won't evaluate the assumes above if a there is no behavior *)