buildozer.spec's custom `p4a.source_dir =` option fails with ERROR: You must give at least one requirement to install (see "pip help install")

[jona...@leadersandco.com]

ERROR: You must give at least one requirement to install (see "pip help install")

With this buildozer:

[app]

title = My App
package.name = myapp
package.domain = com.leadersandco
source.dir = ./src/Cross-Platform
source.include_exts = py,png,jpg,kv,atlas,json,ttf
source.include_patterns = src/Cross-Platform/myapp,src/Cross-Platform/leadersandco-python/JLSoftware/Source,img/
version = 0.1
requirements = python3crystax,kivy,click,opencv3
orientation = portrait

# iOS
osx.python_version = 3
osx.kivy_version = 1.9.1

# Android
fullscreen = 0
android.ndk = 10.3.2
android.ndk_path = ~/.buildozer/crystax-ndk-10.3.2
p4a.branch = master
p4a.source_dir = ./tools/p4a
p4a.local_recipes = ./tools/python-for-android/pythonforandroid/recipes

[buildozer]
log_level = 2
warn_on_root = 1

When I run

 buildozer android debug           

I get 

# Check configuration tokens
# Ensure build layout
# Check configuration tokens
# Preparing build
# Check requirements for android
# Run 'dpkg --version'
# Cwd None
b"Debian 'dpkg' package management program version 1.19.0.5 (amd64).\nThis is free software; see the GNU General Public License version 2 or\nlater for copying conditions. There is NO warranty.\n"# Search for Git (git)
#  -> found at /usr/bin/git
# Search for Cython (cython)
#  -> found at /usr/bin/cython
# Search for Java compiler (javac)
#  -> found at /usr/lib/jvm/java-8-openjdk-amd64/bin/javac
# Search for Java keytool (keytool)
#  -> found at /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/keytool
# Install platform
# Run 'pip install -q --user '
# Cwd None
ERROR: You must give at least one requirement to install (see "pip help install")
# Command failed: pip install -q --user
#
# Buildozer failed to execute the last command
# The error might be hidden in the log above this error
# Please read the full log, and search for it before
# raising an issue with buildozer itself.
# In case of a bug report, please add a full log with log_level = 2

[jona...@leadersandco.com]

The error goes away if I just comment out p4a.source_dir = ./tools/p4a