diff --git a/src/bc-shamir.h b/src/bc-shamir.h
index e787854..0342aac 100644
--- a/src/bc-shamir.h
+++ b/src/bc-shamir.h
@@ -12,8 +12,6 @@
 extern "C" {
 #endif
 
-#include "hazmat.h"
-#include "interpolate.h"
 #include "shamir.h"
 
 #ifdef __cplusplus
diff --git a/src/interpolate.c b/src/interpolate.c
index 0d741fc..6e187de 100644
--- a/src/interpolate.c
+++ b/src/interpolate.c
@@ -32,7 +32,7 @@
 *   values[i] =  | |   -------------
 *              j != i  (xc[i]-xc[j])
 */
-void
+static void
 hazmat_lagrange_basis(uint8_t *values,
                    uint8_t n,
                    const uint8_t *xc,
diff --git a/src/interpolate.h b/src/interpolate.h
index baefb99..0adf37a 100644
--- a/src/interpolate.h
+++ b/src/interpolate.h
@@ -10,27 +10,6 @@
 
 #include "hazmat.h"
 
-/*
- * calculate the lagrange basis coefficients for the lagrange polynomial
- * defined byt the x coordinates xc at the value x.
- *
- * inputs: values: pointer to an array to write the values
- *         n: number of points - length of the xc array, 0 < n <= 32
- *         xc: array of x components to use as interpolating points
- *         x: x coordinate to evaluate lagrange polynomials at
- *
- * After the function runs, the values array should hold data satisfying
- * the following:
- *                ---     (x-xc[j])
- *   values[i] =  | |   -------------
- *              j != i  (xc[i]-xc[j])
- */
-void
-hazmat_lagrange_basis(uint8_t *values,
-                   uint8_t n,
-                   const uint8_t *xc,
-                   uint8_t x);
-
 /**
  * safely interpolate the polynomial going through
  * the points (x0 [y0_0 y0_1 y0_2 ... y0_31]) , (x1 [y1_0 ...]), ...
diff --git a/src/shamir-constants.h b/src/shamir-constants.h
index 95283ef..b2d3e8d 100644
--- a/src/shamir-constants.h
+++ b/src/shamir-constants.h
@@ -17,5 +17,6 @@
 #define SHAMIR_ERROR_CHECKSUM_FAILURE      (-104)
 #define SHAMIR_ERROR_SECRET_TOO_SHORT      (-105)
 #define SHAMIR_ERROR_SECRET_NOT_EVEN_LEN   (-106)
+#define SHAMIR_ERROR_INVALID_THRESHOLD     (-107)
 
 #endif /* SHAMIR_CONSTANTS_H */
diff --git a/src/shamir.c b/src/shamir.c
index e176e3b..1128001 100644
--- a/src/shamir.c
+++ b/src/shamir.c
@@ -51,6 +51,8 @@ int32_t split_secret(
 ) {
     if(shard_count > SHAMIR_MAX_SHARD_COUNT) {
         return SHAMIR_ERROR_TOO_MANY_SHARDS;
+    } else if(threshold < 1 || threshold > shard_count) {
+        return SHAMIR_ERROR_INVALID_THRESHOLD;
     } else if(secret_length > SHAMIR_MAX_SECRET_SIZE) {
         return SHAMIR_ERROR_SECRET_TOO_LONG;
     } else if(secret_length < SHAMIR_MIN_SECRET_SIZE) {
diff --git a/src/shamir.h b/src/shamir.h
index c2caf39..729ffb4 100644
--- a/src/shamir.h
+++ b/src/shamir.h
@@ -43,7 +43,7 @@ uint8_t* create_digest(
  *
  * returns: the number of shards created, or a negative value if there was an error
  *
- * inputs: threshold: number of shards required to recover secret
+ * inputs: threshold: number of shards required to recover secret. Must be 1 <= threshold <= shard_count.
  *         shard_count: number of shards to generate
  *         secret: array of bytes representing the secret
  *         secret_length: length of the secret array. must be >= 16, <= 32 and even.