diff --git a/src/commands.cc b/src/commands.cc
index fbedc529..e66c043e 100644
--- a/src/commands.cc
+++ b/src/commands.cc
@@ -1816,7 +1816,7 @@ const CommandDesc map_key_cmd = {
             throw runtime_error("only a single key can be mapped");
 
         KeymapMode lower_case_only_modes[] = {KeymapMode::Goto};
-        if (key[0].codepoint().map(iswupper).value_or(true) and
+        if (key[0].codepoint().map(iswupper).value_or(false) and
             contains(lower_case_only_modes, keymap_mode))
             throw runtime_error("mode only supports lower case mappings");
 
diff --git a/test/regression/4521-alt-mapping-broken-in-lower-case-modes/cmd b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/cmd
new file mode 100644
index 00000000..ca1112b5
--- /dev/null
+++ b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/cmd
@@ -0,0 +1 @@
+g<a-X>d
diff --git a/test/regression/4521-alt-mapping-broken-in-lower-case-modes/in b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/in
new file mode 100644
index 00000000..257cc564
--- /dev/null
+++ b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/in
@@ -0,0 +1 @@
+foo
diff --git a/test/regression/4521-alt-mapping-broken-in-lower-case-modes/out b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/out
new file mode 100644
index 00000000..56a2a5ae
--- /dev/null
+++ b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/out
@@ -0,0 +1 @@
+fo
diff --git a/test/regression/4521-alt-mapping-broken-in-lower-case-modes/rc b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/rc
new file mode 100644
index 00000000..95c82b04
--- /dev/null
+++ b/test/regression/4521-alt-mapping-broken-in-lower-case-modes/rc
@@ -0,0 +1 @@
+map global goto <a-X> l