Crystal - Make a blank field zero rather than blank

My crystal report is retrieving values from MS Access via ODBC. Occasionally these (numeric) values will be blank. How can I ensure that a zero is displayed in all such fields?

I have tried:

Code:

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If {Field} = "" then
0.00

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But it keeps saying that it's expecting a numeric value here (at the "")

If IsNull({Field}) Then 0 Else {Field}