IsNull For Decimal Acting Up

When I run this code on a column of the type float or real it's ok, but not if the column is decimal, why?

this produces an error:

SELECT IsNull(column1,'') as column1
FROM mytable

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this code works:

SELECT IsNull(column1,0) as column1
FROM mytable

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The problem is that this query is used in a C++ environment to build an insert into another table. The query is build together in a CString.
C++ considers a CString to have ended if it encounters a NULL, therefore I need the check on all columns.

The error message I get is:

"Error converting data type varchar to numeric."

Why is it ok to use float or rel, but not decimal??