diff --git a/lib/oper1.g b/lib/oper1.g
index 143fe218e51..68cc16b8070 100644
--- a/lib/oper1.g
+++ b/lib/oper1.g
@@ -147,6 +147,10 @@ BIND_GLOBAL( "INSTALL_METHOD_FLAGS",
     function( opr, info, rel, flags, baserank, method )
     local   methods,  narg,  i,  k,  tmp, replace, match, j, lk, rank;
 
+    if IS_IDENTICAL_OBJ(opr, method) then
+        Error("Cannot install an operation as a method for itself");
+    fi;
+
     if IsHPCGAP then
         # TODO: once the GAP compiler supports 'atomic', use that
         # to replace the explicit locking and unlocking here.
diff --git a/tst/testbugfix/2021-03-25-InstallMethod.tst b/tst/testbugfix/2021-03-25-InstallMethod.tst
new file mode 100644
index 00000000000..be4bc9da739
--- /dev/null
+++ b/tst/testbugfix/2021-03-25-InstallMethod.tst
@@ -0,0 +1,6 @@
+# Installing an operation as a method for itself is forbidden (now, at least;
+# it used to lead to a segfault if you ever managed to trigger that "method")
+# Fixes GitHub issues #1286 and #4340.
+gap> foo := NewOperation("foo", [IsObject]);;
+gap> InstallMethod(foo, [IsInt], foo);
+Error, Cannot install an operation as a method for itself