diff --git a/nmigen/lib/fifo.py b/nmigen/lib/fifo.py
index ae1ed2d..ad8172f 100644
--- a/nmigen/lib/fifo.py
+++ b/nmigen/lib/fifo.py
@@ -58,7 +58,7 @@ class FIFOInterface:
     w_attributes="",
     r_attributes="")
 
-    def __init__(self, width, depth, fwft):
+    def __init__(self, width, depth, *, fwft):
         self.width = width
         self.depth = depth
         self.fwft  = fwft
@@ -121,8 +121,8 @@ class SyncFIFO(Elaboratable, FIFOInterface):
     """.strip(),
     w_attributes="")
 
-    def __init__(self, width, depth, fwft=True):
-        super().__init__(width, depth, fwft)
+    def __init__(self, width, depth, *, fwft=True):
+        super().__init__(width, depth, fwft=fwft)
 
         self.level = Signal.range(depth + 1)
 
diff --git a/nmigen/test/test_lib_fifo.py b/nmigen/test/test_lib_fifo.py
index 0b7daaa..972afcb 100644
--- a/nmigen/test/test_lib_fifo.py
+++ b/nmigen/test/test_lib_fifo.py
@@ -46,7 +46,7 @@ class FIFOModel(Elaboratable, FIFOInterface):
     Non-synthesizable first-in first-out queue, implemented naively as a chain of registers.
     """
     def __init__(self, width, depth, fwft, rdomain, wdomain):
-        super().__init__(width, depth, fwft)
+        super().__init__(width, depth, fwft=fwft)
 
         self.rdomain = rdomain
         self.wdomain = wdomain