diff --git a/oxifmt.typ b/oxifmt.typ
index 6b8d9e7..c1be2b0 100644
--- a/oxifmt.typ
+++ b/oxifmt.typ
@@ -609,9 +609,21 @@ parameter := argument '$'
   let num-replacements = replacements.pos()
   let named-replacements = replacements.named()
   let unnamed-format-index = 0
-  let fmt-decimal-separator = if "fmt-decimal-separator" in named-replacements { named-replacements.at("fmt-decimal-separator") } else { auto }
-  let fmt-thousands-count = if "fmt-thousands-count" in named-replacements { named-replacements.at("fmt-thousands-count") } else { 3 }
-  let fmt-thousands-separator = if "fmt-thousands-separator" in named-replacements { named-replacements.at("fmt-thousands-separator") } else { "" }
+  let fmt-decimal-separator = auto
+  let fmt-thousands-count = 3
+  let fmt-thousands-separator = ""
+
+  for (name, value) in named-replacements {
+    if name == "fmt-decimal-separator" {
+      fmt-decimal-separator = named-replacements.remove(name)
+    } else if name == "fmt-thousands-count" {
+      fmt-thousands-count = named-replacements.remove(name)
+    } else if name == "fmt-thousands-separator" {
+      fmt-thousands-separator = named-replacements.remove(name)
+    } else if name.starts-with("fmt-") {
+      assert(false, message: "String formatter error: unknown format option '" + name + "'. Keys prefixed with 'fmt-' are reserved for future oxifmt options. Please use a different key name.")
+    }
+  }
 
   assert(
     type(fmt-thousands-count) == _int-type,
diff --git a/tests/strfmt-tests.typ b/tests/strfmt-tests.typ
index 6295c45..b02503a 100644
--- a/tests/strfmt-tests.typ
+++ b/tests/strfmt-tests.typ
@@ -42,9 +42,6 @@
 
   // test custom decimal separators (II) - weird values
   assert.eq(strfmt("{}; {:015e}; {}; {}; {:?}", 1.532, 45000, -5.6, "a.b", "c.d", fmt-decimal-separator: (a: 5)), "1(a: 5)532; 000004(a: 5)5e4; -5(a: 5)6; a.b; \"c.d\"")
-
-  // test custom decimal separators (III) - ensure we can fetch it from inside
-  assert.eq(strfmt("5{fmt-decimal-separator}6", fmt-decimal-separator: "|"), "5|6")
 }
 // Issue #6: UTF-8
 #{