diff --git a/src/blob.c b/src/blob.c
index d758beb..f020966 100644
--- a/src/blob.c
+++ b/src/blob.c
@@ -337,6 +337,36 @@
 }
 
 /*
+ * Check if "n1"- is a valid index for a blobl with length "bloblen".
+ */
+    int
+check_blob_index(long bloblen, varnumber_T n1, int is_range, int quiet)
+{
+    if (n1 < 0 || n1 > bloblen)
+    {
+	if (!quiet)
+	    semsg(_(e_blobidx), n1);
+	return FAIL;
+    }
+    return OK;
+}
+
+/*
+ * Check if "n1"-"n2" is a valid range for a blob with length "bloblen".
+ */
+    int
+check_blob_range(long bloblen, varnumber_T n1, varnumber_T n2, int quiet)
+{
+    if (n2 < 0 || n2 >= bloblen || n2 < n1)
+    {
+	if (!quiet)
+	    semsg(_(e_blobidx), n2);
+	return FAIL;
+    }
+    return OK;
+}
+
+/*
  * Set bytes "n1" to "n2" (inclusive) in "dest" to the value of "src".
  * Caller must make sure "src" is a blob.
  * Returns FAIL if the number of bytes does not match.
diff --git a/src/errors.h b/src/errors.h
index d38562c..99f3aaf 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -401,3 +401,5 @@
 	INIT(= N_("E1181: Cannot use an underscore here"));
 EXTERN char e_blob_required[]
 	INIT(= N_("E1182: Blob required"));
+EXTERN char e_cannot_use_range_with_assignment_operator_str[]
+	INIT(= N_("E1183: Cannot use a range with an assignment operator: %s"));
diff --git a/src/eval.c b/src/eval.c
index 99768ee..d8607f2 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1175,12 +1175,9 @@
 		lp->ll_n1 = (long)tv_get_number(&var1);
 	    clear_tv(&var1);
 
-	    if (lp->ll_n1 < 0
-		    || lp->ll_n1 > bloblen
-		    || (lp->ll_range && lp->ll_n1 == bloblen))
+	    if (check_blob_index(bloblen, lp->ll_n1, lp->ll_range, quiet)
+								       == FAIL)
 	    {
-		if (!quiet)
-		    semsg(_(e_blobidx), lp->ll_n1);
 		clear_tv(&var2);
 		return NULL;
 	    }
@@ -1188,14 +1185,9 @@
 	    {
 		lp->ll_n2 = (long)tv_get_number(&var2);
 		clear_tv(&var2);
-		if (lp->ll_n2 < 0
-			|| lp->ll_n2 >= bloblen
-			|| lp->ll_n2 < lp->ll_n1)
-		{
-		    if (!quiet)
-			semsg(_(e_blobidx), lp->ll_n2);
+		if (check_blob_range(bloblen, lp->ll_n1, lp->ll_n2, quiet)
+								       == FAIL)
 		    return NULL;
-		}
 	    }
 	    lp->ll_blob = lp->ll_tv->vval.v_blob;
 	    lp->ll_tv = NULL;
diff --git a/src/proto/blob.pro b/src/proto/blob.pro
index 7da269c..0b7cbd3 100644
--- a/src/proto/blob.pro
+++ b/src/proto/blob.pro
@@ -14,6 +14,8 @@
 char_u *blob2string(blob_T *blob, char_u **tofree, char_u *numbuf);
 blob_T *string2blob(char_u *str);
 int blob_slice_or_index(blob_T *blob, int is_range, varnumber_T n1, varnumber_T n2, int exclusive, typval_T *rettv);
+int check_blob_index(long bloblen, varnumber_T n1, int is_range, int quiet);
+int check_blob_range(long bloblen, varnumber_T n1, varnumber_T n2, int quiet);
 int blob_set_range(blob_T *dest, long n1, long n2, typval_T *src);
 void blob_remove(typval_T *argvars, typval_T *rettv);
 /* vim: set ft=c : */
diff --git a/src/testdir/test_blob.vim b/src/testdir/test_blob.vim
index 34ed0ca..3699f3b 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -76,16 +76,47 @@
   END
   call CheckLegacyAndVim9Success(lines)
 
-  " TODO: move to above once it works
-  let b = 0zDEADBEEF
-  call assert_fails('let b[2 : 3] = 0z112233', 'E972:')
-  call assert_fails('let b[2 : 3] = 0z11', 'E972:')
-  call assert_fails('let b[3 : 2] = 0z', 'E979:')
+  let lines =<< trim END
+      VAR b = 0zDEADBEEF
+      LET b[2 : 3] = 0z112233
+  END
+  call CheckLegacyAndVim9Failure(lines, 'E972:')
 
-  call assert_fails('let b ..= 0z33', 'E734:')
-  call assert_fails('let b ..= "xx"', 'E734:')
-  call assert_fails('let b += "xx"', 'E734:')
-  call assert_fails('let b[1 : 1] ..= 0z55', 'E734:')
+  let lines =<< trim END
+      VAR b = 0zDEADBEEF
+      LET b[2 : 3] = 0z11
+  END
+  call CheckLegacyAndVim9Failure(lines, 'E972:')
+
+  let lines =<< trim END
+      VAR b = 0zDEADBEEF
+      LET b[3 : 2] = 0z
+  END
+  call CheckLegacyAndVim9Failure(lines, 'E979:')
+
+  let lines =<< trim END
+      VAR b = 0zDEADBEEF
+      LET b ..= 0z33
+  END
+  call CheckLegacyAndVim9Failure(lines, ['E734:', 'E1019:', 'E734:'])
+
+  let lines =<< trim END
+      VAR b = 0zDEADBEEF
+      LET b ..= "xx"
+  END
+  call CheckLegacyAndVim9Failure(lines, ['E734:', 'E1019:', 'E734:'])
+
+  let lines =<< trim END
+      VAR b = 0zDEADBEEF
+      LET b += "xx"
+  END
+  call CheckLegacyAndVim9Failure(lines, ['E734:', 'E1012:', 'E734:'])
+
+  let lines =<< trim END
+      VAR b = 0zDEADBEEF
+      LET b[1 : 1] ..= 0z55
+  END
+  call CheckLegacyAndVim9Failure(lines, ['E734:', 'E1183:', 'E734:'])
 endfunc
 
 func Test_blob_get_range()
diff --git a/src/testdir/vim9.vim b/src/testdir/vim9.vim
index e05f859..9efa655 100644
--- a/src/testdir/vim9.vim
+++ b/src/testdir/vim9.vim
@@ -144,8 +144,23 @@
   try
     exe 'so ' .. fname
     call Func()
-    delfunc! Func
   finally
+    delfunc! Func
+    call chdir(cwd)
+    call delete(fname)
+  endtry
+endfunc
+
+" Check that "lines" inside a legacy function results in the expected error
+func CheckLegacyFailure(lines, error)
+  let cwd = getcwd()
+  let fname = 'XlegacyFails' .. s:sequence
+  let s:sequence += 1
+  call writefile(['func Func()'] + a:lines + ['endfunc', 'call Func()'], fname)
+  try
+    call assert_fails('so ' .. fname, a:error)
+  finally
+    delfunc! Func
     call chdir(cwd)
     call delete(fname)
   endtry
@@ -168,3 +183,35 @@
   CheckDefSuccess(vim9lines)
   CheckScriptSuccess(['vim9script'] + vim9lines)
 enddef
+
+" Execute "lines" in a legacy function, :def function and Vim9 script.
+" Use 'VAR' for a declaration.
+" Use 'LET' for an assignment
+" Use ' #"' for a comment
+def CheckLegacyAndVim9Failure(lines: list<string>, error: any)
+  var legacyError: string
+  var defError: string
+  var scriptError: string
+
+  if type(error) == type('string')
+    legacyError = error
+    defError = error
+    scriptError = error
+  else
+    legacyError = error[0]
+    defError = error[1]
+    scriptError = error[2]
+  endif
+
+  var legacylines = lines->mapnew((_, v) =>
+  				v->substitute('\<VAR\>', 'let', 'g')
+		           	 ->substitute('\<LET\>', 'let', 'g')
+		           	 ->substitute('#"', ' "', 'g'))
+  CheckLegacyFailure(legacylines, legacyError)
+
+  var vim9lines = lines->mapnew((_, v) =>
+  				v->substitute('\<VAR\>', 'var', 'g')
+		           	 ->substitute('\<LET ', '', 'g'))
+  CheckDefExecFailure(vim9lines, defError)
+  CheckScriptFailure(['vim9script'] + vim9lines, scriptError)
+enddef
diff --git a/src/version.c b/src/version.c
index 6ad07f7..0127c2c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2765,
+/**/
     2764,
 /**/
     2763,
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 026a9ee..bdf4a02 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2278,8 +2278,18 @@
 			    if (error)
 				status = FAIL;
 			    else
-				status = blob_set_range(tv_dest->vval.v_blob,
-								   n1, n2, tv);
+			    {
+				long	bloblen = blob_len(tv_dest->vval.v_blob);
+
+				if (check_blob_index(bloblen,
+						       n1, TRUE, FALSE) == FAIL
+					|| check_blob_range(bloblen,
+							n1, n2, FALSE) == FAIL)
+				    status = FAIL;
+				else
+				    status = blob_set_range(
+					     tv_dest->vval.v_blob, n1, n2, tv);
+			    }
 			}
 		    }