diff --git a/mats/foreign.ms b/mats/foreign.ms
index 930eec642..aefc07b92 100644
--- a/mats/foreign.ms
+++ b/mats/foreign.ms
@@ -3680,12 +3680,35 @@
   (error? (make-reference-bytevector -1))
   (error? (bytevector-reference-ref #vu8(1 2 3) 0))
   (error? (bytevector-reference-ref (make-reference-bytevector 8) -8))
+  (< 3 (foreign-sizeof 'ptr)) ;; assumption for some of the following tests
+  (error? (bytevector-reference-ref (make-reference-bytevector 3) 1))
+  (error? (bytevector-reference-ref (make-reference-bytevector 3) 0))
+  (error? (bytevector-reference-ref (make-reference-bytevector 3) -1))
+  (error? (guard (x [#t (error 'bytevector-reference-ref "failed as expected")])
+                 ;; guard the exception to hide the size of the vector, which
+                 ;; varies with the machine's word size
+                 (bytevector-reference-ref (make-reference-bytevector (foreign-sizeof 'ptr)) 1)))
   (error? (bytevector-reference-ref (make-reference-bytevector 8) 'oops))
   (error? (bytevector-reference*-ref (make-reference-bytevector 8) -8))
   (error? (bytevector-reference*-ref (make-reference-bytevector 8) 'oops))
+  (error? (bytevector-reference*-ref (make-reference-bytevector 3) 1))
+  (error? (bytevector-reference*-ref (make-reference-bytevector 3) 0))
+  (error? (bytevector-reference*-ref (make-reference-bytevector 3) -1))
+  (error? (guard (x [#t (error 'bytevector-reference-ref "failed as expected")])
+                 (bytevector-reference-ref (make-reference*-bytevector (foreign-sizeof 'ptr)) 1)))
+  (error? (bytevector-reference-set! (make-reference-bytevector 8) -8 #false))
+  (error? (bytevector-reference-set! (make-reference-bytevector 8) 'oops #false))
+  (error? (bytevector-reference-set! (make-reference-bytevector 3) 1 #false))
+  (error? (bytevector-reference-set! (make-reference-bytevector 3) 0 #false))
+  (error? (guard (x [#t (error 'bytevector-reference-set! "failed as expected")])
+                 (bytevector-reference-set! (make-reference-bytevector  (foreign-sizeof 'ptr)) 1 #false)))
   (error? (reference-address->object #f))
   (error? (reference*-address->object #f))
 
+  (equal? #false (bytevector-reference-ref (make-reference-bytevector (foreign-sizeof 'ptr)) 0))
+  (equal? #false (bytevector-reference*-ref (make-reference-bytevector (foreign-sizeof 'ptr)) 0))
+  (equal? (void) (bytevector-reference-set! (make-reference-bytevector (foreign-sizeof 'ptr)) 0 #false))
+
   (not (reference-bytevector? #vu8(1 2 3)))
   (not (reference-bytevector? 7))
   (begin
diff --git a/mats/root-experr-compile-0-f-f-f b/mats/root-experr-compile-0-f-f-f
index b8df4f0b1..6fd56e566 100644
--- a/mats/root-experr-compile-0-f-f-f
+++ b/mats/root-experr-compile-0-f-f-f
@@ -10458,9 +10458,22 @@ foreign.mo:Expected error in mat collect-safe: "foreign-callable: string result
 foreign.mo:Expected error in mat reference-bytevector: "make-reference-bytevector: -1 is not a valid bytevector length".
 foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: #vu8(1 2 3) is not a reference bytevector".
 foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: invalid index -8 for #vu8(0 0 0 0 0 0 ...)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: invalid index 1 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: invalid index 0 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: invalid index -1 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: failed as expected".
 foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: invalid index oops for #vu8(0 0 0 0 0 0 ...)".
 foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference*-ref: invalid index -8 for #vu8(0 0 0 0 0 0 ...)".
 foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference*-ref: invalid index oops for #vu8(0 0 0 0 0 0 ...)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference*-ref: invalid index 1 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference*-ref: invalid index 0 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference*-ref: invalid index -1 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-ref: failed as expected".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-set!: invalid index -8 for #vu8(0 0 0 0 0 0 ...)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-set!: invalid index oops for #vu8(0 0 0 0 0 0 ...)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-set!: invalid index 1 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-set!: invalid index 0 for #vu8(0 0 0)".
+foreign.mo:Expected error in mat reference-bytevector: "bytevector-reference-set!: failed as expected".
 foreign.mo:Expected error in mat reference-bytevector: "reference-address->object: invalid address #f".
 foreign.mo:Expected error in mat reference-bytevector: "reference*-address->object: invalid address #f".
 ftype.mo:Expected error in mat ftype: "unexpected function ftype outside pointer field (function #f (int) int)".
diff --git a/s/prims.ss b/s/prims.ss
index fb925609d..14aa49064 100644
--- a/s/prims.ss
+++ b/s/prims.ss
@@ -88,7 +88,7 @@
   (lambda (bv i)
     (unless (reference-bytevector? bv) ($oops who "~s is not a reference bytevector" bv))
     (unless (and (fixnum? i)
-                 (not ($fxu< (fx- (bytevector-length bv) (fx- (constant ptr-bytes) 1)) i)))
+                 (fx<= 0 i (fx- (bytevector-length bv) (constant ptr-bytes))))
       ($oops who "invalid index ~s for ~s" i bv))
     (bytevector-reference-ref bv i)))
 
@@ -97,7 +97,7 @@
     (lambda (bv i)
       (unless (reference-bytevector? bv) ($oops who "~s is not a reference bytevector" bv))
       (unless (and (fixnum? i)
-                   (not ($fxu< (fx- (bytevector-length bv) (fx- (constant ptr-bytes) 1)) i)))
+                   (fx<= 0 i (fx- (bytevector-length bv) (constant ptr-bytes))))
         ($oops who "invalid index ~s for ~s" i bv))
       (ref bv i))))
 
@@ -105,7 +105,7 @@
   (lambda (bv i val)
     (unless (reference-bytevector? bv) ($oops who "~s is not a reference bytevector" bv))
     (unless (and (fixnum? i)
-                 (not ($fxu< (fx- (bytevector-length bv) (fx- (constant ptr-bytes) 1)) i)))
+                 (fx<= 0 i (fx- (bytevector-length bv) (constant ptr-bytes))))
       ($oops who "invalid index ~s for ~s" i bv))
     (bytevector-reference-set! bv i val)))