diff --git a/changelog.d/20260713_144500_fromstring_fn4.md b/changelog.d/20260713_144500_fromstring_fn4.md
new file mode 100644
index 0000000..0e5235a
--- /dev/null
+++ b/changelog.d/20260713_144500_fromstring_fn4.md
@@ -0,0 +1,7 @@
+### Fixed
+
+- `fromStringImpl` matches its `Fn4` declaration: the Lua entry is a single
+  4-ary function (previously it was a chain of nested single-argument
+  closures, so `Data.Number.fromString` returned a function instead of a
+  `Maybe Number` — Lua silently drops the surplus arguments of the n-ary
+  `runFn4` call).
diff --git a/src/Data/Number.lua b/src/Data/Number.lua
index 5242d3c..fc922ff 100644
--- a/src/Data/Number.lua
+++ b/src/Data/Number.lua
@@ -42,20 +42,15 @@ return {
   sqrt = (math.sqrt),
   tan = (math.tan),
   trunc = (function(x) return x < 0 and math.ceil(x) or math.floor(x) end),
-  fromStringImpl = (function(str)
-    return function(isFinite)
-      return function(just)
-        return function(nothing)
-          -- Mirror JS parseFloat: a leading-numeric prefix is parsed and
-          -- trailing junk (and a whitespace prefix) is tolerated. Guard against
-          -- a failed parse — the old code wrapped tonumber's nil in Just — and
-          -- use the supplied isFinite predicate so non-finite results are Nothing.
-          local prefix = str:match("^%s*([%-+]?%d*%.?%d+[eE]?[%-+]?%d*)")
-          local x = prefix and tonumber(prefix) or nil
-          if x ~= nil and isFinite(x) then return just(x) end
-          return nothing
-        end
-      end
-    end
+  fromStringImpl = (function(str, isFinite, just, nothing)
+    -- Declared Fn4, so the entry is a single 4-ary function.
+    -- Mirror JS parseFloat: a leading-numeric prefix is parsed and
+    -- trailing junk (and a whitespace prefix) is tolerated. Guard against
+    -- a failed parse — the old code wrapped tonumber's nil in Just — and
+    -- use the supplied isFinite predicate so non-finite results are Nothing.
+    local prefix = str:match("^%s*([%-+]?%d*%.?%d+[eE]?[%-+]?%d*)")
+    local x = prefix and tonumber(prefix) or nil
+    if x ~= nil and isFinite(x) then return just(x) end
+    return nothing
   end)
 }
diff --git a/test/regression/number.lua b/test/regression/number.lua
index d118ca9..9a6d688 100644
--- a/test/regression/number.lua
+++ b/test/regression/number.lua
@@ -50,10 +50,12 @@ check("min 1 NaN is NaN", isNaN(M.min(1)(nan)), tostring(M.min(1)(nan)))
 check("min NaN 1 is NaN", isNaN(M.min(nan)(1)), tostring(M.min(nan)(1)))
 check("min 3 7 == 3", M.min(3)(7) == 3, tostring(M.min(3)(7)))
 
--- #93 fromString (via the Fn4-style curried FFI)
+-- #93 fromString. fromStringImpl is declared `Fn4`, so the Lua entry is a
+-- 4-ary function called with all arguments at once (`runFn4` and the
+-- compiler's uncurried lift both call it that way).
 local function just(x) return {tag = "just", value = x} end
 local nothing = {tag = "nothing"}
-local function fromString(s) return M.fromStringImpl(s)(M.isFinite)(just)(nothing) end
+local function fromString(s) return M.fromStringImpl(s, M.isFinite, just, nothing) end
 local function isJust(m, v) return type(m) == "table" and m.tag == "just" and m.value == v end
 local function isNothing(m) return type(m) == "table" and m.tag == "nothing" end