diff --git a/basis/bootstrap/image/primitives/primitives.factor b/basis/bootstrap/image/primitives/primitives.factor
index 7c149b9db9..5ffd047ad6 100644
--- a/basis/bootstrap/image/primitives/primitives.factor
+++ b/basis/bootstrap/image/primitives/primitives.factor
@@ -404,7 +404,7 @@ CONSTANT: all-primitives {
                 "(format-float)" ( n fill width precision format locale -- byte-array )
                 "format_float"
                 { float byte-array fixnum fixnum byte-array byte-array } { byte-array }
-                make-foldable
+                make-flushable
             }
         }
     }
diff --git a/core/math/parser/parser-tests.factor b/core/math/parser/parser-tests.factor
index 4b8bbd1e35..24ccd1aab8 100644
--- a/core/math/parser/parser-tests.factor
+++ b/core/math/parser/parser-tests.factor
@@ -1,4 +1,5 @@
-USING: layouts literals math math.parser.private sequences tools.test ;
+USING: kernel layouts literals math math.parser.private sequences
+strings tools.test ;
 IN: math.parser
 
 { f }
@@ -473,3 +474,16 @@ unit-test
 { "" } [
     33.4 "" 4 4 "f" "missing" format-float
 ] unit-test
+
+! Literal byte arrays are mutable, so (format-float) isn't foldable.
+: trouble ( -- str ba )
+    155000.0 B{ } -1 3 B{ 69 0 } [
+        B{ 67 0 } (format-float) >string
+    ] keep ;
+
+{
+    "1.55E+05"
+    "1.550e+05"
+} [
+    trouble CHAR: e 0 rot set-nth trouble drop
+] unit-test