midas: Hessian has become unstable or asymmetric r(504);

I have one more query on midas please.
While doing analysis, an error statement came up as below
Hessian has become unstable or asymmetric
r(504);

When I clicked on r(504), the following message came up.

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[P] error . . . . . . . . . . . . . . . . . . . . . . . . Return code 504
matrix has missing values
This return code is now infrequently used because, beginning
with version 8, Stata now permits missing values in matrices.

(end of search)
In my database, there are no missing values.
Please help