diff --git a/src/tinyexpr.rs b/src/tinyexpr.rs
index 5d653f638..7c49d2f70 100644
--- a/src/tinyexpr.rs
+++ b/src/tinyexpr.rs
@@ -405,12 +405,14 @@ impl<'s> State<'s> {
             // Look for a function call.
             // But not when it's an "x" followed by whitespace
             // - that's the alternative multiplication operator.
-            if next.first()?.is_ascii_lowercase()
+            // We look for alphabetic here even tho all our function names are ASCII,
+            // in order to give a nicer error.
+            if next.first()?.is_alphabetic()
                 && !(*next.first()? == 'x' && next.len() > 1 && next[1].is_whitespace())
             {
                 let ident_len = next
                     .iter()
-                    .position(|&c| !(c.is_ascii_lowercase() || c.is_ascii_digit() || c == '_'))
+                    .position(|&c| !(c.is_alphabetic() || c.is_ascii_digit() || c == '_'))
                     .unwrap_or(next.len());
 
                 let ident = &next[..ident_len];
diff --git a/tests/checks/math.fish b/tests/checks/math.fish
index 712160c5c..cb2c4bec3 100644
--- a/tests/checks/math.fish
+++ b/tests/checks/math.fish
@@ -127,6 +127,12 @@ not math 'blah()'
 # CHECKERR: 'blah()'
 # CHECKERR:  ^~~^
 
+# There is also no "Blah" function.
+not math 'Blah()'
+# CHECKERR: math: Error: Unknown function
+# CHECKERR: 'Blah()'
+# CHECKERR:  ^~~^
+
 math n + 4
 # CHECKERR: math: Error: Unknown function
 # CHECKERR: 'n + 4'