diff --git a/README.md b/README.md
index 1e7d590..bdba58f 100644
--- a/README.md
+++ b/README.md
@@ -100,10 +100,10 @@ to show the usage of the driver program (reported below for convenience).
 		For example, it could be the de Bruijn graph topology output by BCALM.
 	
 	 k
-		K-mer length (must be <= 32).
+		K-mer length (must be <= 31).
 	
 	 m
-		Minimizer length (must be <= k).
+		Minimizer length (must be < k).
 	
 	 [-s seed]
 		Seed for construction (default is 1).
diff --git a/include/builder/build.cpp b/include/builder/build.cpp
index e0c83bd..3efcca0 100644
--- a/include/builder/build.cpp
+++ b/include/builder/build.cpp
@@ -448,7 +448,7 @@ void dictionary::build(std::string const& filename, build_configuration const& b
                                  " but got k = " + std::to_string(build_config.k));
     }
     if (build_config.m == 0) throw std::runtime_error("m must be > 0");
-    if (build_config.m > build_config.k) throw std::runtime_error("m must be <= k");
+    if (build_config.m > build_config.k) throw std::runtime_error("m must be < k");
     if (build_config.l > constants::max_l) {
         throw std::runtime_error("l must be <= " + std::to_string(constants::max_l));
     }
diff --git a/include/util.hpp b/include/util.hpp
index 61b96b2..6440e6e 100644
--- a/include/util.hpp
+++ b/include/util.hpp
@@ -10,7 +10,7 @@
 namespace sshash {
 
 namespace constants {
-constexpr uint64_t max_k = 32;
+constexpr uint64_t max_k = 31;  // max *odd* size that can be packed into 64 bits
 constexpr uint64_t invalid = uint64_t(-1);
 constexpr uint64_t seed = 1;
 constexpr uint64_t hashcode_bits = 64;
diff --git a/src/build.cpp b/src/build.cpp
index ac76395..9d690e4 100644
--- a/src/build.cpp
+++ b/src/build.cpp
@@ -18,7 +18,7 @@ int main(int argc, char** argv) {
                "\t- one DNA sequence per line.\n"
                "\tFor example, it could be the de Bruijn graph topology output by BCALM.");
     parser.add("k", "K-mer length (must be <= " + std::to_string(constants::max_k) + ").");
-    parser.add("m", "Minimizer length (must be <= k).");
+    parser.add("m", "Minimizer length (must be < k).");
 
     /* optional arguments */
     parser.add("seed",