Then this is the expected behavior but not a bug.
iGrid automatically converts the edited text to a value of the data type of the cell value. Thus, to "fix" the problem, you need to have the corresponding data type in your cell before you edit your cells. If you give us a source code, we'll show how to correct it.
FYI: You can also use the BeforeCommitEdit event to handle this situation if you need your custom error message or want to suppress it at all. Here is the corresponding excerpt from the CHM help file from the topic dedicated to BeforeCommitEdit:
iGrid automatically converts the edited text in a text box cell to a value of the cell value data type. iGrid shows its standard error message if the user tries to save illegal value in a text box cell and the SilentValidation property is set to False; if this property is set to True, you can programmatically detect the conversion error in this event using the lConvErr parameter. If the error has occurred, this parameter contains a non-zero value which is a standard VB error code.