diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index 546643192..5f2de3bdb 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -1298,6 +1298,29 @@ class CustomTarget(Target):
                 deps.append(c)
         return deps
 
+    def flatten_command(self, cmd):
+        if not isinstance(cmd, list):
+            cmd = [cmd]
+        final_cmd = []
+        for c in cmd:
+            if hasattr(c, 'held_object'):
+                c = c.held_object
+            if isinstance(c, (str, File)):
+                final_cmd.append(c)
+            elif isinstance(c, dependencies.ExternalProgram):
+                if not c.found():
+                    m = 'Tried to use not-found external program {!r} in "command"'
+                    raise InvalidArguments(m.format(c.name))
+                final_cmd += c.get_command()
+            elif isinstance(c, (BuildTarget, CustomTarget)):
+                self.dependencies.append(c)
+                final_cmd.append(c)
+            elif isinstance(c, list):
+                final_cmd += self.flatten_command(c)
+            else:
+                raise InvalidArguments('Argument {!r} in "command" is invalid'.format(c))
+        return final_cmd
+
     def process_kwargs(self, kwargs):
         super().process_kwargs(kwargs)
         self.sources = kwargs.get('input', [])
@@ -1325,32 +1348,7 @@ class CustomTarget(Target):
             if os.path.split(depfile)[1] != depfile:
                 raise InvalidArguments('Depfile must be a plain filename without a subdirectory.')
             self.depfile = depfile
-        cmd = kwargs['command']
-        if not(isinstance(cmd, list)):
-            cmd = [cmd]
-        final_cmd = []
-        for i, c in enumerate(cmd):
-            if hasattr(c, 'held_object'):
-                c = c.held_object
-            if isinstance(c, (str, File)):
-                final_cmd.append(c)
-            elif isinstance(c, dependencies.ExternalProgram):
-                if not c.found():
-                    raise InvalidArguments('Tried to use not found external program {!r} in a build rule.'.format(c.name))
-                final_cmd += c.get_command()
-            elif isinstance(c, (BuildTarget, CustomTarget)):
-                self.dependencies.append(c)
-                final_cmd.append(c)
-            elif isinstance(c, list):
-                # Hackety hack, only supports one level of flattening. Should really
-                # work to arbtrary depth.
-                for s in c:
-                    if not isinstance(s, str):
-                        raise InvalidArguments('Array as argument %d contains a non-string.' % i)
-                    final_cmd.append(s)
-            else:
-                raise InvalidArguments('Argument %s in "command" is invalid.' % i)
-        self.command = final_cmd
+        self.command = self.flatten_command(kwargs['command'])
         if self.capture:
             for c in self.command:
                 if isinstance(c, str) and '@OUTPUT@' in c:
diff --git a/test cases/common/56 custom target/meson.build b/test cases/common/56 custom target/meson.build
index fd59fbd80..2e6f69cf9 100644
--- a/test cases/common/56 custom target/meson.build	
+++ b/test cases/common/56 custom target/meson.build	
@@ -8,11 +8,13 @@ endif
 # Note that this will not add a dependency to the compiler executable.
 # Code will not be rebuilt if it changes.
 comp = '@0@/@1@'.format(meson.current_source_dir(), 'my_compiler.py')
+# Test that files() in command: works. The compiler just discards it.
+useless = files('installed_files.txt')
 
 mytarget = custom_target('bindat',
 output : 'data.dat',
 input : 'data_source.txt',
-command : [python, comp, '--input=@INPUT@', '--output=@OUTPUT@'],
+command : [python, comp, '--input=@INPUT@', '--output=@OUTPUT@', useless],
 install : true,
 install_dir : 'subdir'
 )
diff --git a/test cases/common/56 custom target/my_compiler.py b/test cases/common/56 custom target/my_compiler.py
index 4ba2da64e..f46d23a91 100755
--- a/test cases/common/56 custom target/my_compiler.py	
+++ b/test cases/common/56 custom target/my_compiler.py	
@@ -1,16 +1,21 @@
 #!/usr/bin/env python3
 
+import os
 import sys
 
+assert(os.path.exists(sys.argv[3]))
+
+args = sys.argv[:-1]
+
 if __name__ == '__main__':
-    if len(sys.argv) != 3 or not sys.argv[1].startswith('--input') or \
-       not sys.argv[2].startswith('--output'):
-        print(sys.argv[0], '--input=input_file --output=output_file')
+    if len(args) != 3 or not args[1].startswith('--input') or \
+       not args[2].startswith('--output'):
+        print(args[0], '--input=input_file --output=output_file')
         sys.exit(1)
-    with open(sys.argv[1].split('=')[1]) as f:
+    with open(args[1].split('=')[1]) as f:
         ifile = f.read()
     if ifile != 'This is a text only input file.\n':
         print('Malformed input')
         sys.exit(1)
-    with open(sys.argv[2].split('=')[1], 'w') as ofile:
+    with open(args[2].split('=')[1], 'w') as ofile:
         ofile.write('This is a binary output file.\n')