Too little is known about the true cost of bringing in a law on the Welsh language, a group of assembly members says.
The proposed Welsh language measure would place duties on some firms to provide services in Welsh.
But the Finance Committee said the assembly government's figures give little explanation of some funding.
It is understood most of the costs will be met by the £13.8m that currently supports the Welsh Language Board.
The board would be dissolved under the new legislation and replaced with a Welsh language commissioner.
Among the commissioner's duties will be the promotion of equality between Welsh and English.
But a report from the committee says that an explanatory memorandum accompanying the proposed law - known as a measure - identifies a number of new and changed activities which will need funding.
But few details are given of their cost, it said.
'Sums don't add up'
Angela Burns AM, chair of the finance committee, called on the assembly government to ensure the measure stands up to "robust and intricate financial scrutiny".
"The committee sees a concerning lack of substantive information for the cost implications of the Welsh language measure," she said.
"I'm afraid that with the limited research and lack of detail contained within the explanatory memorandum the Welsh Government's sums, at the moment, don't add up.
"The evidence my committee colleagues and I received from the commercial sector demonstrated the likelihood of significant funding to meet the requirements of the proposed measure, but the poor detail businesses have to go on makes it difficult to establish a true cost."
The proposed new law on the Welsh language was published by the assembly government in March.
Under the measure, firms in areas such as telecoms, gas and electricity would face sanctions, including fines, if they fail to meet language service delivery standards.