diff --git a/.gitattributes b/.gitattributes
index c3d77bec09..5bc3a2b377 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -9730,6 +9730,7 @@ tests/tbs/tb0583a.pp svneol=native#text/plain
 tests/tbs/tb0584.pp svneol=native#text/pascal
 tests/tbs/tb0585.pp svneol=native#text/pascal
 tests/tbs/tb0586.pp svneol=native#text/pascal
+tests/tbs/tb0587.pp svneol=native#text/plain
 tests/tbs/tb205.pp svneol=native#text/plain
 tests/tbs/ub0060.pp svneol=native#text/plain
 tests/tbs/ub0069.pp svneol=native#text/plain
diff --git a/compiler/x86/rax86.pas b/compiler/x86/rax86.pas
index ec7bd5e1dd..66cf6ed496 100644
--- a/compiler/x86/rax86.pas
+++ b/compiler/x86/rax86.pas
@@ -789,6 +789,10 @@ begin
                   case tx86operand(operands[2]).opsize of
                     S_L :
                       opsize:=S_WL;
+{$ifdef x86_64}
+                    S_Q :
+                      opsize:=S_WQ;
+{$endif}
                   end;
                 S_B :
                   begin
@@ -797,6 +801,10 @@ begin
                         opsize:=S_BW;
                       S_L :
                         opsize:=S_BL;
+{$ifdef x86_64}
+                      S_Q :
+                        opsize:=S_BQ;
+{$endif}
                     end;
                   end;
               end;
diff --git a/tests/tbs/tb0587.pp b/tests/tbs/tb0587.pp
new file mode 100644
index 0000000000..fb36c3ed34
--- /dev/null
+++ b/tests/tbs/tb0587.pp
@@ -0,0 +1,29 @@
+{ %cpu=x86_64 }
+{ %opt=-Aas }
+
+{$asmmode intel}
+
+procedure test;
+var
+  i1,i2: int64;
+  b: shortint;
+begin
+  b:=-128;
+  asm
+    movsx rax, b
+    mov  i1,rax
+    movzx rax, b
+    mov  i2,rax
+  end ['rax'];
+  if i1<>-128 then
+    halt(1);
+  if i2<>byte(-128) then
+    halt(2);
+end;
+
+begin
+  test
+end.
+    
+  
+