引文“”在第 X 页上未定义在第 Y 行上

Question

我有一个 references.bib 文件，其中有很多参考文献，即：

@article{karl,
    title = {{Motion Planning for Parking a Truck and Trailer System}},
    year = {2019},
    journal = { Master Thesis KTH},
    author = {Karl Kvarnfors}
}

@article{differential_flatness,
    title = {{Flatness and defect of non-linear systems: Introductory theory and examples}},
    year = {1995},
    journal = {International Journal of Control, 61(6)},
    author = {M. Fliess, J. Lévine, P. Martin, and P. Rouchon}
}

如果我\cite{karl}这样做，它会绘制 [4]，但如果\cite{differential_flatness}它绘制 [差分平坦度]。 你知道为什么吗？