diff --git a/src/lsp/format.lsp b/src/lsp/format.lsp
index cc2ab778013d0d94ce5f0b7d9cb24d9e892833a2..07ca0c08e6575a1664dc0b2d379ccc5d161be196 100644
--- a/src/lsp/format.lsp
+++ b/src/lsp/format.lsp
@@ -273,7 +273,8 @@
                  (schar string posn))))
       (loop
          (let ((char (get-char)))
-           (cond ((or (char<= #\0 char #\9) (char= char #\+) (char= char #\-))
+           (cond ((and (not colonp) (not atsignp)
+                       (or (char<= #\0 char #\9) (char= char #\+) (char= char #\-)))
                   (multiple-value-bind
                         (param new-posn)
                       (parse-integer string :start posn :junk-allowed t)
@@ -285,7 +286,8 @@
                        (decf posn))
                       (t
                        (return)))))
-                 ((or (char= char #\v) (char= char #\V))
+                 ((and (not colonp) (not atsignp)
+                       (or (char= char #\v) (char= char #\V)))
                   (push (cons posn :arg) params)
                   (incf posn)
                   (case (get-char)
@@ -294,7 +296,8 @@
                      (decf posn))
                     (t
                      (return))))
-                 ((char= char #\#)
+                 ((and (not colonp) (not atsignp)
+                       (char= char #\#))
                   (push (cons posn :remaining) params)
                   (incf posn)
                   (case (get-char)
@@ -303,13 +306,15 @@
                      (decf posn))
                     (t
                      (return))))
-                 ((char= char #\')
+                 ((and (not colonp) (not atsignp)
+                       (char= char #\'))
                   (incf posn)
                   (push (cons posn (get-char)) params)
                   (incf posn)
                   (unless (char= (get-char) #\,)
 		   (decf posn)))
-                 ((char= char #\,)
+                 ((and (not colonp) (not atsignp)
+                       (char= char #\,))
                   (push (cons posn nil) params))
                  ((char= char #\:)
                   (if colonp
diff --git a/src/tests/normal-tests/mixed.lsp b/src/tests/normal-tests/mixed.lsp
index c189acb2a2858ed38f16402a86c2b0f78a335248..b52c7bf1d708a08fd28c84c88281ba07349e76af 100644
--- a/src/tests/normal-tests/mixed.lsp
+++ b/src/tests/normal-tests/mixed.lsp
@@ -466,3 +466,16 @@
   (is (eql (realpart (log -2s0 2l0)) 1l0))
   (is (eql (log 2d0 2l0) 1l0))
   (is (eql (realpart (log -2d0 2l0)) 1l0)))
+
+;;; Created: 2023-01-07
+;;; Contains: tests checking for illegal format parameters that occur
+;;; after at signs or colons.
+(test mix.0025.illegal-format-parameters
+  (signals error (format nil "a~@4A" nil))
+  (signals error (format nil "a~:4A" nil))
+  (signals error (format nil "a~:@4A" nil))
+  (signals error (format nil "a~@:4A" nil))
+  (is (equal (format nil "a~4@A" nil) "a NIL"))
+  (is (equal (format nil "a~4:A" nil) "a()  "))
+  (is (equal (format nil "a~4:@A" nil) "a  ()"))
+  (is (equal (format nil "a~4@:A" nil) "a  ()")))