diff --git a/spot/tl/ltlf.cc b/spot/tl/ltlf.cc
index 7a07a3b2b..8e9d3d64c 100644
--- a/spot/tl/ltlf.cc
+++ b/spot/tl/ltlf.cc
@@ -30,6 +30,8 @@ namespace spot
       switch (auto o = f.kind())
         {
         case op::strong_X:
+          o = op::X;
+          SPOT_FALLTHROUGH;
         case op::F:
           return formula::unop(o, formula::And({alive, t(f[0])}));
         case op::X:             // weak
diff --git a/tests/core/ltlfilt.test b/tests/core/ltlfilt.test
index dd8f85a65..3d79695c7 100755
--- a/tests/core/ltlfilt.test
+++ b/tests/core/ltlfilt.test
@@ -335,7 +335,7 @@ al & X(a | !al) & (al U G!al)
 al & F(a & al & !X(a | !al) & X(!al | b)) & (al U G!al)
 a & al & (b | c) & (al U G!al)
 0
-al & ((a & b) U (al & !X[!]al)) & (al U G!al)
+al & ((a & b) U (al & !Xal)) & (al U G!al)
 EOF
 
 checkopt --ltl --from-ltlf='!dead' <<EOF
@@ -350,7 +350,7 @@ checkopt --ltl --from-ltlf='!dead' <<EOF
 !dead & F(a & !dead & !X(a | dead) & X(b | dead)) & (!dead U Gdead)
 a & !dead & (b | c) & (!dead U Gdead)
 0
-!dead & ((a & b) U (!dead & !X[!]!dead)) & (!dead U Gdead)
+!dead & ((a & b) U (!dead & !X!dead)) & (!dead U Gdead)
 EOF
 
 cat >in <<EOF