This error occurs when both and appear in an ordinary differential equation (ODE), resulting in an ambiguous equation which could mean (for instance) that either the ODE  or the ODE is used.

Example 1
Specifying an equation containing both the function  and the constant/parameter is ambiguous because it is not clear whether the ODE  or is intended to be used.

Solution:

Remove the ambiguity by replacing with .

Example 2

Similarly, in the following example, it is unclear whether the ODE is meant to be  or

Solution:

Remove the ambiguity by replacing in with .

dsolve, diff