diff --git a/sphinx/writers/latex.py b/sphinx/writers/latex.py
index 140ee2136..56b3736db 100644
--- a/sphinx/writers/latex.py
+++ b/sphinx/writers/latex.py
@@ -277,11 +277,12 @@ def escape_abbr(text):
 
 def rstdim_to_latexdim(width_str):
     """Convert `width_str` with rst length to LaTeX length."""
-    match = re.match('(\d*\.?\d*)\s*(\S*)', width_str)
+    match = re.match('^(\d*\.?\d*)\s*(\S*)$', width_str)
     if not match:
         raise ValueError
     res = width_str
     amount, unit = match.groups()[:2]
+    float(amount)  # validate amount is float
     if not unit:
         return None
     elif unit == 'pt':
diff --git a/tests/test_writer_latex.py b/tests/test_writer_latex.py
index 4ef1cdda8..667b5e98d 100644
--- a/tests/test_writer_latex.py
+++ b/tests/test_writer_latex.py
@@ -11,6 +11,8 @@
 from __future__ import print_function
 from sphinx.writers.latex import rstdim_to_latexdim
 
+from util import raises
+
 
 def test_rstdim_to_latexdim():
     # Length units docutils supported
@@ -26,3 +28,7 @@ def test_rstdim_to_latexdim():
     assert rstdim_to_latexdim('160') is None
 
     assert rstdim_to_latexdim('160.0em') == '160.0em'
+
+    # unknown values (it might be generated by 3rd party extension)
+    raises(ValueError, rstdim_to_latexdim, 'unknown')
+    assert rstdim_to_latexdim('160.0unknown') == '160.0unknown'