diff --git a/assemble.c b/assemble.c
index a1a90f2..c9fd31e 100644
--- a/assemble.c
+++ b/assemble.c
@@ -1474,6 +1474,12 @@ static int assemble(
                                 free_tree(value);
                                 return 0;
                             }
+
+                            if ((mode.type & 070) == 0) {
+                                report(stack->top, "JSR Rn,Rm is illegal\n");
+                                /* But encode it anyway... */
+                            }
+
                             word = op->value | mode.type | (reg << 6);
                             store_word(stack->top, tr, 2, word);
                             mode_extension(tr, &mode, stack->top);
diff --git a/tests/test-jmp.mac b/tests/test-jmp.mac
index d9b0ca9..b578a90 100644
--- a/tests/test-jmp.mac
+++ b/tests/test-jmp.mac
@@ -4,9 +4,13 @@
 ; JMP Rx is not allowed, all other modes are.
 ;
 
-start:  jmp     end
+start:  jmp     label           ; rel(pc)       jumps to label
+        jmp     @label          ; @rel(pc)      does something else!
+        jmp     #label          ; (pc)+         does something else!
+        jmp     @#label         ; @(pc)+        jumps to label
+
         jmp     r1              ; must fail
-        jmp     (r1)
+        jmp     (r2)
         jmp     (r1)+
         jmp     @(r1)+
         jmp     -(r1)
@@ -14,4 +18,18 @@ start:  jmp     end
         jmp     1234(r1)
         jmp     @1234(r1)
 
-end:
+secnd:  jsr     r0,label        ; rel(pc)       jumps to label
+        jsr     r0,@label       ; @rel(pc)      does something else!
+        jsr     r0,#label       ; (pc)+         does something else!
+        jsr     r0,@#label      ; @(pc)+        jumps to label
+
+        jsr     r2,r1           ; must fail
+        jsr     r3,(r2)
+        jsr     r4,(r1)+
+        jsr     r5,@(r1)+
+        jsr     r2,-(r1)
+        jsr     r3,@-(r1)
+        jsr     r4,1234(r1)
+        jsr     r5,@1234(r1)
+
+label:  rts     pc