TiKZ: trouble with angles ! Missing \endcsname inserted.

Question

I'm trying to draw the arc of an angle between two lines in tikz. I'm using the angles library and have pretty much copied the example from the manual, but when I try to label the arc, I get the following error:

! Missing \endcsname inserted.
<to be read again>
                   \theta
l.29 ...c [draw, "$\theta$", angle eccentricity=3]{angle=P--V--Q};

I am following the syntax from this answer. But I have the same trouble using the syntax from the manual pg 570.

Removing the label allows me to draw the arc no trouble. But I would really like to put a theta in my diagram. I thought it might be an escaping issue so I tried labelling with "$t$", and then "t". In the last case the error tells me tikz doesn't know of the label t.

my drawing code is below

\documentclass[border=20pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc,angles}
\begin{document}
\begin{tikzpicture}

  \def \radius{3.5}
  \coordinate (V) at ($(0,0)!\radius cm!rand*45:(0,\radius)$);

  \path (-\radius, 0) coordinate(P) -- 
               coordinate[midway](O)
               (\radius, 0) coordinate(Q);

  % Draw semicircle
  \draw (Q) arc(0:180:\radius1);
  \draw[shorten <=-5mm, shorten >=-5mm] (P) -- (Q);

  %draw an arc starting at [partway from V to Q], to[ partway from V to P]
  \draw [color=blue] (P)--(V)--(Q);
  \pic [draw, "$\theta$", angle eccentricity=3] {angle=P--V--Q};


  \node at (O) [below]  {$O$};
  \node at (P) [below]  {$P$};
  \node at (Q) [below]  {$Q$};
  \node at (V) [above]  {$V$};

  \foreach \p in {O,P,Q, V}{
    \fill (\p) circle (1pt);
  }\

\end{tikzpicture}


\end{document}

Answer 1

Your problem is solved by @Roland comment, so here are some off-topic suggestions:

\documentclass[border=20pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{angles,
                calc,
                quotes} % <---
\begin{document}
    \begin{tikzpicture}
\def \radius{3.5}
\coordinate (V) at ($(0,0)!\radius cm!rand*45:(0,\radius)$);
\path (-\radius, 0) coordinate[label=below:$P$] (P)  % <---
                --  coordinate[label=below:$O$] (O) (\radius, 0) % <---
                    coordinate[label=below:$Q$] (Q); % <---
% draw semicircle
\draw (Q) arc(0:180:\radius);
\draw[shorten <=-5mm, shorten >=-5mm] (P) -- (Q);
% draw triangles's cathetus
\draw [color=blue, semithick] (P)--(V)--(Q);
% draw angle
\pic [draw=blue, text=red, "$\theta$", angle eccentricity=1.5] {angle=P--V--Q}; % <---
% dots

\foreach \p in {O,P,Q, V}{\fill (\p) circle[radius=1.5pt];}
    \end{tikzpicture}
\end{document}

Edit: colored angle as you desired in your comment.

Answer 2

Even if the previous answer is such a clean solution, I think you would definitely discover the tkz-euclide package which is extremely versatile when it comes to drawing mathematical geometric figures. I didn't rewrite all your figure with tkz-euclid commands, but you could save time by doing it for your future projects. Here, the only change is for drawing and labeling the angle.

\documentclass[border=20pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc,angles}
\usepackage{tkz-euclide}
\begin{document}
\begin{tikzpicture}

  \def \radius{3.5}
  \coordinate (V) at ($(0,0)!\radius cm!rand*45:(0,\radius)$);

  \path (-\radius, 0) coordinate(P) -- 
               coordinate[midway](O)
               (\radius, 0) coordinate(Q);

  % Draw semicircle
  \draw (Q) arc(0:180:\radius1);
  \draw[shorten <=-5mm, shorten >=-5mm] (P) -- (Q);

  %draw an arc starting at [partway from V to Q], to[ partway from V to P]
  \draw [color=blue] (P)--(V)--(Q);

  %-------------- Here's the use of tkz-euclide-----
  \tkzMarkAngle[color=blue,mark=none,size=.5](P,V,Q)
  \tkzLabelAngle[color=red,pos=0.8](P,V,Q){$\theta$}
  %-------------------------------------------------


  \node at (O) [below]  {$O$};
  \node at (P) [below]  {$P$};
  \node at (Q) [below]  {$Q$};
  \node at (V) [above]  {$V$};

  \foreach \p in {O,P,Q, V}{
    \fill (\p) circle (1pt);
  }\

\end{tikzpicture}


\end{document}